Fixing documentation for XML::Node#namespace as described by this bug report:
Also added tests for XML::Node#namespace when invoked on attribute nodes.
Correcting documentation for #namespace method.
Adds tests for #namespace on attribute nodes.
Travis build failure seems to be unrelated to these changes. They fail on some other tests and only when run under JRuby, so I don't think I can fix it.
Close #803. Fix #802.
Fix the documentation of XML::Node#namespace and add more tests to test
the behavior of namespaces