Allow trailing whitespace for complex messages

[eemeli]

Fix for a part of #809, CC @lucacasonato

When parsing a complex message, we should be lenient about trailing whitespace, and silently ignore it. It has no meaning, and should not be considered an error.

None of the parser complexity concerns around leading whitespace apply here.

[eemeli]

This is good, but I think we should also do leading whitespace. From my implementation experience, this is a trivial to implement and can be implemented to be as fast as the current spec behavior.

I completely agree, but I'd like to ensure that we at least get trailing whitespace rather than having this blocked by implementation or other concerns about mode detection with leading whitespace.