Enable ping for http/2
Merge pull request #471 from adriancole/http2-ping

Enable ping for http/2
Backfilled http/2 data frame test and clarified comments.
Don't unzip if there isn't a response body.

https://github.com/square/okhttp/issues/358
Merge pull request #474 from square/jwilson_0121_gzip

Don't unzip if there isn't a response body.
Merge pull request #472 from adriancole/backfill-data-test

Backfilled http/2 data frame test and clarified comments.
Remove FLAG_END_FLOW_CONTROL as it is no longer in http/2.  Backfill test and implementation window update on http/2.
Add GOAWAY support to http/2 and backfill tests.
Merge pull request #476 from adriancole/http2-goaway

Add HTTP/2 GOAWAY frame
convert http/2 params to natural primitive types.
Merge pull request #477 from adriancole/ids-and-fields

frame metadata field cleanup