Fix trailing space removal (wikitext, 13e622d)

This was an off-by-one error that was harmless under normal operation because both code paths ended up returning the same length.

But when running under "space_to_underscore" mode it was enough to actually prevent the trailing space removal from working in the case where there was only one trailing space.

It worked when "space_to_underscore" was false only because each space was actually three characters (%20) so the test passed anyway.

Signed-off-by: Greg Hurrell <greg@hurrell.net>