Add support for http2 based requests

[phouri]

http2 module just landed in nodejs, it'd be great to add support for http2 tests via supertest.

[ravichandranjv]

Yes, changing the superagent to include https does not make any difference either, the spdy stream may have to be integrated (or readable) with supertest. Edit: I used spdy and not http2 because http2 does not seem to be fully supported in Express.