Extract links from XHTML documents with MIME-Type "application/xml"

"application/xhtml+xml" is already interpreted as HTML
and link extractor is fine with it to extract links.

Only for XML documents can the namespaces in tags be an issue.

Fixes #780

Do both tag and attribute tests in _iter_links() method
Merge pull request #791 from redapple/xhtmllinks

Extract links from XHTML documents with MIME-Type "application/xml"