This revert #202 (commit 9d37f4c) because trying to parse PSR-7 request
was a mistake. The whole point of PSR-7 is to allow for interoperability
and be able to use specialized libs for body parsing (amongst many other
things). Trying to parse ourselves would be opening a can of worm if/when
other content types have to be supported. It is more correct and future
safe to require that the body is parsed before being passed to GraphQL.
