Split out the Document#createElementNS tests from DOMImplementation-createDocument.html.
Rewrite Document-createElementNS.html to also run the tests originally written for DOMImplementation#createDocument.
Add some tests for null/undefined/empty qualified name arguments to Document#createElementNS.
Add some more Document#createElementNS tests.
Merge pull request #1306 from Ms2ger/createDocument-createElementNS

Share tests for createElementNS and createDocument; r=plh