changed content-type 'x-www-form-urlencoded' to valid content-type 'appl...

Hi,
I changed content-type 'x-www-form-urlencoded' to valid content-type 'application/x-www-form-urlencoded'

Content type for response should be 'application/x-www-form-urlencoded'. 
Some client libraries are using this to check the response format.

This is most likely a breaking change for most users of the library so it may be better to include this in a 0.2.0 release or to add it as an option to createServer()