if node doesnt have parent unlink in jruby should return self.

[esse]

If node doesn't have parent node - simply return self, as would Nokogiri for C would do, because that means that node is already detached (test case included).

This commit fixes issue #1112.

[flavorjones]

Thanks for submitting this! I've cleaned up the implementation with a subsequent commit, and added more explicit test coverage in test/xml/test_unparented_node.rb.