Fix invalid return expressions in functions returning void
Fix a tokenizer bug causing incorrect handling of duplicate attributes

The tag buffer was retaining duplicate attribute values that were marked
to be discarded, thus clobbering the following attribute name.

This also fixes the relevant test, which didn't catch this particular
case because it had an empty (boolean-style) value as the duplicate.
Merge pull request #67 from craigbarnes/dup-attr-fix

Fix tokenizer bug
Merge pull request #61 from craigbarnes/voidfix

Fix invalid return expressions in functions returning void