Core::Response cleanup

in this PR I did:
- the 'serializer' attribute is now immutable (as it is set only when Response object is created).
- fixed a bug: the status code return by Core::HTTP::status was not properly checking it for a numeric value (only anchored digit sequence to the beginning of the given scalar)
- cleaned up the module (removed redundant imports and modules loaded) + made the code nicer (well, that's arguable)
-  added some missing documentation for some methods