Add tests for SPARQL results parsing.

These commits add tests for the `parse_x_bindings` functions, and fix a few minor issues found along the way:

- update nodes hash when parsing json and tsv.
- support empty bindings in tsv results.

It was a bit tricky to wedge these into rspec; I'm open to suggestions. If nothing else this can be used to check the correctness of #36.

Thanks!