PayloadParser refactor

Need to refactor a few more tests. Otherwise, this is done.
Ok found the catch – the original `CanonicalPayloadFixture` did not contain the trailing newlines (as they are produced by the Sundown parser) see https://github.com/apiaryio/snowcrash/blob/master/test/test-PayloadParser.cc#L58

So this change and behavior is actually correct.