XPath queries do not work with namespaces and fixed encoding property

Hello (again!)

I noticed that any XPath queries containing namespaces `//ns1:tagname` would fail with the error:
```
XPath error : Undefined namespace prefix
XPath error : Invalid expression
```

I've added a test case for this issue and the Travis build file to ensure the tests on my fork pass :)
https://travis-ci.org/rhodgkins/Ono/builds/26876794