Convert to RSpec 3 Syntax.

I try to run the spec but get following error either invoking `bundle exec rake` or `bundle exec rake spec` or `rake` or `rake spec`:

```
➜  http git:(master) bundle exec rake
/Users/juan/.rubies/ruby-2.1.2/bin/ruby -I/Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib:/Users/juan/.gem/ruby/2.1.2/gems/rspec-support-3.0.4/lib -S /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/exe/rspec ./spec/http/authorization_header/basic_auth_spec.rb ./spec/http/authorization_header/bearer_token_spec.rb ./spec/http/authorization_header_spec.rb ./spec/http/backports/base64_spec.rb ./spec/http/backports/uri_spec.rb ./spec/http/client_spec.rb ./spec/http/content_type_spec.rb ./spec/http/headers/mixin_spec.rb ./spec/http/headers_spec.rb ./spec/http/options/body_spec.rb ./spec/http/options/form_spec.rb ./spec/http/options/headers_spec.rb ./spec/http/options/json_spec.rb ./spec/http/options/merge_spec.rb ./spec/http/options/new_spec.rb ./spec/http/options/proxy_spec.rb ./spec/http/options_spec.rb ./spec/http/redirector_spec.rb ./spec/http/request/writer_spec.rb ./spec/http/request_spec.rb ./spec/http/response/body_spec.rb ./spec/http/response_spec.rb ./spec/http_spec.rb
/Users/juan/.gem/ruby/2.1.2/gems/simplecov-html-0.8.0/lib/simplecov-html.rb:58: warning: possibly useless use of a variable in void context
[2014-08-20 23:40:03] INFO  WEBrick 1.3.1
[2014-08-20 23:40:03] INFO  ruby 2.1.2 (2014-05-08) [x86_64-darwin13.0]
[2014-08-20 23:40:03] INFO  WEBrick::HTTPServer#start: pid=40206 port=65432
[2014-08-20 23:40:03] INFO  WEBrick 1.3.1
[2014-08-20 23:40:03] INFO  ruby 2.1.2 (2014-05-08) [x86_64-darwin13.0]
[2014-08-20 23:40:03] INFO  WEBrick 1.3.1
[2014-08-20 23:40:03] INFO  ruby 2.1.2 (2014-05-08) [x86_64-darwin13.0]
[2014-08-20 23:40:03] INFO  WEBrick::HTTPProxyServer#start: pid=40206 port=8080
[2014-08-20 23:40:03] INFO  WEBrick::HTTPProxyServer#start: pid=40206 port=8081
Coverage report generated for RSpec to /Users/juan/dev/http/coverage. 326 / 638 LOC (51.1%) covered.
[Coveralls] Outside the Travis environment, not sending data.
[2014-08-20 23:40:04] INFO  going to shutdown ...
[2014-08-20 23:40:04] INFO  WEBrick::HTTPProxyServer#start done.
[2014-08-20 23:40:04] INFO  going to shutdown ...
[2014-08-20 23:40:04] INFO  WEBrick::HTTPProxyServer#start done.
[2014-08-20 23:40:04] INFO  going to shutdown ...
[2014-08-20 23:40:04] INFO  WEBrick::HTTPServer#start done.
/Users/juan/dev/http/spec/http/content_type_spec.rb:7:in `block (3 levels) in <top (required)>': undefined method `its' for RSpec::ExampleGroups::HTTPContentType::Parse::WithTextPlain:Class (NoMethodError)
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/example_group.rb:331:in `module_exec'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/example_group.rb:331:in `subclass'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/example_group.rb:227:in `block in define_example_group_method'
	from /Users/juan/dev/http/spec/http/content_type_spec.rb:5:in `block (2 levels) in <top (required)>'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/example_group.rb:331:in `module_exec'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/example_group.rb:331:in `subclass'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/example_group.rb:227:in `block in define_example_group_method'
	from /Users/juan/dev/http/spec/http/content_type_spec.rb:4:in `block in <top (required)>'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/example_group.rb:331:in `module_exec'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/example_group.rb:331:in `subclass'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/example_group.rb:227:in `block in define_example_group_method'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/dsl.rb:41:in `block in expose_example_group_alias'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/dsl.rb:79:in `block (2 levels) in expose_example_group_alias_globally'
	from /Users/juan/dev/http/spec/http/content_type_spec.rb:3:in `<top (required)>'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/configuration.rb:1058:in `load'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/configuration.rb:1058:in `block in load_spec_files'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/configuration.rb:1058:in `each'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/configuration.rb:1058:in `load_spec_files'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/runner.rb:97:in `setup'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/runner.rb:85:in `run'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/runner.rb:70:in `run'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib/rspec/core/runner.rb:38:in `invoke'
	from /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/exe/rspec:4:in `<main>'
/Users/juan/.rubies/ruby-2.1.2/bin/ruby -I/Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/lib:/Users/juan/.gem/ruby/2.1.2/gems/rspec-support-3.0.4/lib -S /Users/juan/.gem/ruby/2.1.2/gems/rspec-core-3.0.4/exe/rspec ./spec/http/authorization_header/basic_auth_spec.rb ./spec/http/authorization_header/bearer_token_spec.rb ./spec/http/authorization_header_spec.rb ./spec/http/backports/base64_spec.rb ./spec/http/backports/uri_spec.rb ./spec/http/client_spec.rb ./spec/http/content_type_spec.rb ./spec/http/headers/mixin_spec.rb ./spec/http/headers_spec.rb ./spec/http/options/body_spec.rb ./spec/http/options/form_spec.rb ./spec/http/options/headers_spec.rb ./spec/http/options/json_spec.rb ./spec/http/options/merge_spec.rb ./spec/http/options/new_spec.rb ./spec/http/options/proxy_spec.rb ./spec/http/options_spec.rb ./spec/http/redirector_spec.rb ./spec/http/request/writer_spec.rb ./spec/http/request_spec.rb ./spec/http/response/body_spec.rb ./spec/http/response_spec.rb ./spec/http_spec.rb failed
```

so I convert the spec using transpec. Now I can run spec and all tests are passed.