Kill all mutants in Zero::Request::Parameter

I've also changed "extract_query_params" because it does not matter, if the
query string it empty or not for the "parse_string" method.
Kill all mutants i Zero::Request

Fix tests for PATCH, because spec_helper didn't support it and the tests where
wrong.
Remove if statement from "conent_type", because ruby already returns nil, if the
requested key in a hash is not found. (Sounds a bit ugly, but it's true.)