Unit test for #28 PrettyPrinter adds empty attribute with xmlns

	* src/test/scala/scala/xml/XMLTest.scala (issue28): New unit test
Fix #28 by using TopScope instead of null

	* src/main/scala/com/jonasboner/PrettyPrinter.scala (format):
	Modify namespace binding default argument to TopScope, was null.
Merge pull request #28 from ashawley/pretty-print-ns-defect

PrettyPrinter adds empty attribute with xmlns