Instances are only usable once

[leonerd]

I'm attempting to parse inner contents of a =begin ... block using a helper instance of another Pod::Simple subclass. I find that after I use an instance just once by calling ->parse_string_document on it, any further invocations do nothing. If I want to do another parse, I have to throw it away and start again with a brand new instance. For performance this is a little slow; it'd be nice if I could use the same instance repeatedly.

[khwilliamson]

patches welcome