fix unnecessary warning after adding request with lazy referencing 

the problem is related to validation check of [`request`](https://github.com/apiaryio/snowcrash/blob/master/src/PayloadParser.h#L321) and [`response`](https://github.com/apiaryio/snowcrash/blob/master/src/PayloadParser.h#L346) at the finalization of `PayloadParser`, with lazy referencing being implemented although a response or request might be in pending for late referencing the function would still check if response has empty body(which a response with late referencing does at this stage) so it produce a warning accordingly.
this PR suggest to separate validation in different functions so they can be called after lazy referencing being solved also to check if the status of the given response or request is pending which would stop unnecessary warning.
I do not see how this "checks a validation" – the description and name of these function seems confusing – did you mean something like `checkRequest` (`checkResponse`)