Reset 'each' hash iterator

No one knows which namespace is passed to 'registerNs' method,
because 'each' is called again in 'create_xpath_context' without resetting
'each' hash iterator.

And result of 'each' on hash after insertion without resetting hash
iterator is undefined from Perl 5.18.
Merge pull request #3 from syohex/reset_iterator

Reset 'each' hash iterator