Send header one line at a time from curb adapter

When calling the on_header callback because this is how curb behaves.
net_http.rb: StubSocket needs continue_timeout for aws-sdk

For Ruby 2.0
See:
https://github.com/aws/aws-sdk-ruby/blob/7c4ea3d6004068e4905994be91de459651a96116/lib/aws/core/http/connection_pool.rb#L127

and:
https://github.com/ruby/ruby/blob/v2_0_0_0/lib/net/http.rb#L725
Can't write to a Logger in a signal handler

Remove warning with ruby 2.0.
Warning shows below:
"log writing failed. can't be called from trap context"

Related:
Can't write to a Logger in a signal handler
https://bugs.ruby-lang.org/issues/7917
Allow passing file as the payload body as in:

https://github.com/igrigorik/em-http-request/wiki/Streaming#streaming-data-off-disk
Merge pull request #278 from godfat/continue_timeout

net_http.rb: StubSocket needs continue_timeout for aws-sdk
add a test case for passing :file for em-http-request.
Merge pull request #280 from godfat/em-http-request-file

Allow passing file as the payload body with em-http-request
Use new MiniTest::Test
Use accessor for MiniTest assertions count
Specify MiniTest 5.0.x as development dependency
Merge pull request #281 from timkurvers/minitest-5-compatibility

MiniTest 5 compatibility
Ensured response body is available if `after_request` callbacks even if `response_block` was used is Excon adapter.
Upgraded version check on Excon to `>= 0.22.0`
Only run spec for em-http-request specific behaviour when em-http-request >= 1.0 is loaded.
Minitest 5 and Test::Unit compatibility.
Removed version limit on jruby-openssl
Updated travis config to not allow failures on Ruby 2.0.0 and JRuby 1.9
Merge pull request #279 from sanemat/feature/logger-handling

Can't write to a Logger in a signal handler
Merge pull request #270 from mocoso/call-on-header-one-line-at-a-time

Send header one line at a time from curb adapter
Add Ruby syntax highlighting to code examples in README