Clean up Rack input stream handling

[no-reply]

The parser methods used to try to handle Strings, as well as valid Rack inputs. This led to some unnecessary complexity, giving rise to errors like those seen in #56. This refactors to handle only IO-like Rack Input streams.

Most of the work here is just to reorganize the tests.

There is an odd case where RDFSource#update feeds the IO stream before trying to parse a graph. For now, we aggressively #rewind the stream in #parse_graph to this problem. This might be unduely costly, and there is probably a better fix in the long run.

---

The later commits introduce Rack::Lint and deal with a few issues found by running the existing tests and the W3C suite with those in place.

[no-reply]

Closes #56 and replaces #57.

[no-reply]

Closes #58