MODE-1795 Importing XML should map default XML namespace

When importing XML, any XML namespaces should be used, even if the prefix for the XML namespace matches an existing namespace prefix. Per Section 11.1 of the JSR-283 specification, an existing JCR namespace should be used if its URL matches the XML namespace URI. If there is no existing JCR namespace matching the XML namespace URI, then the namespace should be registered using the XML namespace prefix if not already used or a generated prefix if the XML namespace prefix is already used.

A new unit test case was added to verify the behavior.

This pull request should be merged onto the '3.1.x' branch and cherry-picked to the 'master' branch. See the [related pull-request](https://github.com/ModeShape/modeshape/pull/680) for the changes to the '2.x' branch.