Remove method override support through body content

[amazeika]

Request method overrides through body content currently properly work for application/x-www-form-urlencoded requests only.

This happens since the data request transport is late to the party when decoding and setting the request data object for all other request types.

Supporting this strategy for method override is kind of problematic and would result on code duplication among other problems. Instead we should just support method overrides through headers which is more semantically correct.