Fixed DomNode.lookup_prefix() and lookup_namespace_ns() with Unit Tests

* Fixes require to change some behavior on setting new elements
  by using a prefix in qualifed name as namespace prefix and ns use
  a uri, just in line as DOM4 specification
* Changed Element.set_ns_attr to use a prefix in qualifed name
  for namespace, searching for existing ones or adding a new if no
  one if found
* Fixed a bug in Element.attrs.keys, falling an infinit loop