Exception for POST without content-length

When POSTing through a Rack::Proxy with a `nil` CONTENT_LENGTH, an exception is raised:

    ArgumentError: Content-Length not given and Transfer-Encoding is not `chunked'
        rack-proxy/lib/net_http_hacked.rb:51:in `begin_request_hacked'
        rack-proxy/lib/rack/http_streaming_response.rb:54:in `response'
        rack-proxy/lib/rack/http_streaming_response.rb:14:in `status'
        rack-proxy/lib/rack/proxy.rb:47:in `perform_request'
        rack-proxy/lib/rack/proxy.rb:11:in `call'