XmlNodePrinter does not handle attribute namespaces correctly

The following script fails, because XmlNodePrinter ignores the attribute namespace. The output of writer.toString() is <root

type="foo"/>

def attributeWithNamespaceInput = "<root ns1:type=\"foo\" xmlns:ns1=\"www.foo.bar\"/>\n"
def root = new XmlParser().parseText(attributeWithNamespaceInput)
def writer = new StringWriter()
new XmlNodePrinter(new PrintWriter(writer)).print(root)
assert writer.toString() == attributeWithNamespaceInput

I attached a fix for XmlNodePrinter and updated the corresponding test class.