Tests for operations with revision 0 URI 

I wrote tests for operations with revision 0 URI. Once this pull request https://github.com/PoundPay/balanced/pull/361 is deployed, these tests shall pass.

@mjallday 
I notice that it seems the the `accept` and `content-type` headers [are misplaced](https://github.com/balanced/balanced-python/compare/rev0-uri-tests?expand=1#diff-8486612e8d9134a5776194dd3c5b53b3L26). I saw [ruby client does otherwise](https://github.com/balanced/balanced-ruby/blob/master/lib/balanced/client.rb#L22)