Avoid needing to call jsx:decode/1 more than once per request.

[HernanRivasAcosta]

egithub_webhook:request() should be opaque and have this spec: #{headers => map(), body => map() | binary()}.

Use a function to get the body which always returns a map and if the body in the request is a binary, parses it and returns the new request. Similar to what cowboy does.