Add hpack-test-case test module.

Issue square/okhttp#931
Merge pull request #976 from westarle/931-add-hpack-tests

Add hpack-test-case based tests
Permit GETs to have Content-Length: 0.

It's ridiculous, but Apache HTTP client wants it.
Closes https://github.com/square/okhttp/issues/990
Merge pull request #996 from square/jwilson_0720_content-length-zero

Permit GETs to have Content-Length: 0.