Improve consistency of the OMElement#addAttribute(String, String, OMNamespace) API:
* Let LLOM's addAttribute implementation generate a prefix (instead of throwing a NPE) if the prefix is null and no namespace declaration for the given namespace URI is found.
* Add test coverage for the scenario where the prefix is null and a namespace declaration for the given URI is found (in which case the corresponding prefix is reused). This worked correctly in LLOM.
* Make DOOM's behavior consistent with LLOM.
* Update Javadoc of the addAttribute method.

git-svn-id: https://svn.apache.org/repos/asf/webservices/axiom/trunk@1592903 13f79535-47bb-0310-9956-ffa450edef68
Added some more assertions.

git-svn-id: https://svn.apache.org/repos/asf/webservices/axiom/trunk@1592904 13f79535-47bb-0310-9956-ffa450edef68