ParserCreate(): Allow an empty string for the namespace_separator argument;

ParserCreate():  Allow an empty string for the namespace_separator argument;
    while not generally a good idea, this is used by RDF users, and works
    to implement RDF-style namespace+localname concatenation as defined
    in the RDF specifications.  (This also corrects a backwards-compatibility
    bug.)

Be more conservative while clearing out handlers; set the slot in the
self->handlers array to NULL before DECREFing the callback.

Still more adjustments to make the code style internally consistent.