Nil response body

When using Bullet Rack with Sinatra I get the following stack trace...

```
NoMethodError: undefined method `empty?' for nil:NilClass
        C:/Ruby/jruby-1.7.4/lib/ruby/gems/shared/gems/bullet-4.6.0/lib/bullet/rack.rb:37:in `empty?'
        C:/Ruby/jruby-1.7.4/lib/ruby/gems/shared/gems/bullet-4.6.0/lib/bullet/rack.rb:14:in `call'
        C:/Ruby/jruby-1.7.4/lib/ruby/gems/shared/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/connection_pool.rb:626:in `call'
        C:/Ruby/jruby-1.7.4/lib/ruby/gems/shared/gems/rack-1.5.2/lib/rack/lint.rb:49:in `_call'
        C:/Ruby/jruby-1.7.4/lib/ruby/gems/shared/gems/rack-1.5.2/lib/rack/lint.rb:37:in `call'
        C:/Ruby/jruby-1.7.4/lib/ruby/gems/shared/gems/rack-1.5.2/lib/rack/showexceptions.rb:24:in `call'
        C:/Ruby/jruby-1.7.4/lib/ruby/gems/shared/gems/puma-2.4.0-java/lib/puma/rack_patch.rb:13:in `call'
        C:/Ruby/jruby-1.7.4/lib/ruby/gems/shared/gems/rack-1.5.2/lib/rack/chunked.rb:43:in `call'
        C:/Ruby/jruby-1.7.4/lib/ruby/gems/shared/gems/rack-1.5.2/lib/rack/content_length.rb:14:in `call'
        C:/Ruby/jruby-1.7.4/lib/ruby/gems/shared/gems/puma-2.4.0-java/lib/puma/server.rb:381:in `handle_request'
        C:/Ruby/jruby-1.7.4/lib/ruby/gems/shared/gems/puma-2.4.0-java/lib/puma/server.rb:252:in `process_client'
        C:/Ruby/jruby-1.7.4/lib/ruby/gems/shared/gems/puma-2.4.0-java/lib/puma/server.rb:151:in `run'
        org/jruby/RubyProc.java:255:in `call'
        C:/Ruby/jruby-1.7.4/lib/ruby/gems/shared/gems/puma-2.4.0-java/lib/puma/thread_pool.rb:92:in `spawn_thread'
```
I've tracked this back to [rack.rb](https://github.com/flyerhzm/bullet/blob/master/lib/bullet/rack.rb#L37) where there is no check if ```#response_body``` returns a ```nil``` before ```#empty?``` is called on it. This pull fixes that.

P.s. I would have updated the rack spec to cover this but I am unable to get the specs up and working on my Windows machine