Fix multiple xml:lang attributes error

As discussed in email with @lrowe: when the html tag in the content includes an xml:lang attribute (as it does in Plone 4.2.2), then copying the tag's attributes results in *two* xml:lang attributes in the output.

I've fixed the issue and added a test. All tests pass.

However, I'm not confident my fix is the best way to do it, so please review before merging.