PrettyPrinter adds empty attribute with xmlns

An element with an `xmlns` attribute gets converted to a string correctly, but using the pretty printer to convert the XML to a string adds an extra, empty attribute.  I have no insight on how to fix the pretty printer.

There is a failing unit test provided by this pull request that can reproduce this issue:

    [error] Test scala.xml.XMLTest.issue28 failed: expected:<...x:foo xmlns:x="gaga"[]/>> but was:<...x:foo xmlns:x="gaga"[ xmlns=""]/>>
    :9:19: '/' expected instead of ''                  ^
    :9:19: name expected, but char '' cannot start a name                  ^