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.
Because PSR-7 specification only specify that `getParsedBody()` **may**
return the parsed body for `application/json`, we cannot assume that it
is always the case. So if the value returned parsed body is an empty array,
it means we should try to parse it ourselves (`null` would mean no body at
all according to spec).
With this modification we try to used given parsed body, but fallback on
trying to parse the body if necessary. This leave the door open to custom
implementation of parsing if needed, while making it easier to use out of
the box.
