Bug 70867: Set xmlns namespace URI on elements that reference it

* Creative commons snippets from the xhtml days have a bunch of xmlns
  attributes which are refused when building the dom without a namespace
  uri for the namespace reference.

* Many examples in the bug report. Here's one:
    enwiki/Talk:Marzio_Conti?oldid=561831755

* Added new parser test that crashes without this patch.

Change-Id: I864724e0c48d9d70339d7f723280f4593f67adb0