Add ResponseFormat interface to identify classes with responseFormat setter/getter
Add getEncodedContentType which returns the default, accepted or from responseFormat guessed contentType.
Used to for setting HTTPResponse contentType and getting the encoder.
Merge pull request #104 from CarstenHollmann/fix/contentType_guessing_responseFormat

Fix/content type guessing response format
Add ResposneProxy for resposne header access by ResponseWriters
Merge pull request #105 from shane-axiom/fix-writer-headers

Add ResponseProxy for response header access by ResponseWriters