Default value for has_xpath_value

[ivanwills]

Added ability to set a default value for the has_xpath_value attribute helper. This is required by some features in XMLSchema documents.

[robinsmidsrod]

@ivanwills Sorry for the extremely long wait for a response on this PR. I've looked into it a bit, and the test to validate this code is quite hard to understand. Can you try to simplify it, so it's a bit more obvious what you're testing?

Also, are you trying to store the default you want in the XML file or in the perl code that uses XML::Rabbit? The intent is not clear from the code and documentation you've included.