Bringing Accept header parsing in line with the spec.

For reference:
  http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1
  http://www.w3.org/Protocols/rfc2616/rfc2616-sec2.html#sec2.2
Ensuring that generated Content-Type headers properly escape params.
Exposing Accept header parameters.
Simplifying regular expression to silence warnings.
Merge pull request #626 from pvande/improving-accept-headers

Improving Accept header handling