Use Sanitizer::mergeAttributes() for Linker::linkAttribs(). Also clean up whitespace...

Use Sanitizer::mergeAttributes() for Linker::linkAttribs().  Also clean up whitespace for mergeAttributes and reduce number of nested functions, and don't try to merge non-string 'class' arguments.  This last point is necessary so I can have 'class' => false work right for linkAttribs(), but it makes sense.  Parser tests pass.