Proxy does not support HEAD request

[rledisez]

When issuing a HEAD request through the proxy of sc-server, sc-server dies.

How to reproduce:

1. Set the following directive in a project Buildfile:
   proxy '/sproutcoredotcom', :to => 'sproutcore.com:80', :secure => false
2. Send a HEAD request:
   curl -X HEAD http://localhost:4020/sproutcoredotcom

Result:

FATAL 19:07:24.007 ~ undefined method `call' for nil:NilClass 
FATAL 19:07:24.007 ~ BACKTRACE:
/Users/romain/.gem/ruby/1.9.1/gems/sproutcore-1.11.0/lib/sproutcore/rack/proxy.rb:43:in `block in call'
/Users/romain/.gem/ruby/1.9.1/gems/sproutcore-1.11.0/lib/sproutcore/rack/proxy.rb:42:in `each'
/Users/romain/.gem/ruby/1.9.1/gems/sproutcore-1.11.0/lib/sproutcore/rack/proxy.rb:42:in `call'
/Users/romain/.gem/ruby/1.9.1/gems/sproutcore-1.11.0/lib/sproutcore/rack/proxy.rb:211:in `block (2 levels) in handle_proxy'
/Users/romain/.gem/ruby/1.9.1/gems/eventmachine-1.0.4/lib/em/deferrable.rb:151:in `call'
/Users/romain/.gem/ruby/1.9.1/gems/eventmachine-1.0.4/lib/em/deferrable.rb:151:in `set_deferred_status'
/Users/romain/.gem/ruby/1.9.1/gems/eventmachine-1.0.4/lib/em/deferrable.rb:191:in `succeed'
/Users/romain/.gem/ruby/1.9.1/gems/em-http-request-1.1.1/lib/em-http/client.rb:114:in `unbind'
/Users/romain/.gem/ruby/1.9.1/gems/em-http-request-1.1.1/lib/em-http/client.rb:71:in `on_request_complete'
/Users/romain/.gem/ruby/1.9.1/gems/em-http-request-1.1.1/lib/em-http/http_connection.rb:129:in `block in post_init'
/Users/romain/.gem/ruby/1.9.1/gems/em-http-request-1.1.1/lib/em-http/http_connection.rb:144:in `call'
/Users/romain/.gem/ruby/1.9.1/gems/em-http-request-1.1.1/lib/em-http/http_connection.rb:144:in `<<'
/Users/romain/.gem/ruby/1.9.1/gems/em-http-request-1.1.1/lib/em-http/http_connection.rb:144:in `receive_data'
/Users/romain/.gem/ruby/1.9.1/gems/em-http-request-1.1.1/lib/em-http/http_connection.rb:23:in `receive_data'
/Users/romain/.gem/ruby/1.9.1/gems/eventmachine-1.0.4/lib/eventmachine.rb:187:in `run_machine'
/Users/romain/.gem/ruby/1.9.1/gems/eventmachine-1.0.4/lib/eventmachine.rb:187:in `run'
/Users/romain/.gem/ruby/1.9.1/gems/thin-1.6.3/lib/thin/backends/base.rb:73:in `start'
/Users/romain/.gem/ruby/1.9.1/gems/thin-1.6.3/lib/thin/server.rb:162:in `start'
/Users/romain/.gem/ruby/1.9.1/gems/rack-1.6.0/lib/rack/handler/thin.rb:19:in `run'
/Users/romain/.gem/ruby/1.9.1/gems/sproutcore-1.11.0/lib/sproutcore/rack/service.rb:93:in `start'
/Users/romain/.gem/ruby/1.9.1/gems/sproutcore-1.11.0/lib/sproutcore/tools/server.rb:71:in `server'
/Users/romain/.gem/ruby/1.9.1/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
/Users/romain/.gem/ruby/1.9.1/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
/Users/romain/.gem/ruby/1.9.1/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
/Users/romain/.gem/ruby/1.9.1/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
/Users/romain/.gem/ruby/1.9.1/gems/sproutcore-1.11.0/lib/sproutcore/tools.rb:445:in `start'
/Users/romain/.gem/ruby/1.9.1/gems/sproutcore-1.11.0/lib/sproutcore/tools.rb:30:in `invoke'
/Users/romain/.gem/ruby/1.9.1/gems/sproutcore-1.11.0/bin/sc-server:11:in `<top (required)>'
/Users/romain/.gem/ruby/1.9.1/bin/sc-server:23:in `load'
/Users/romain/.gem/ruby/1.9.1/bin/sc-server:23:in `<main>'