x/net/http2: flow control over payload length, not data length

[bradfitz]

http://httpwg.org/specs/rfc7540.html#rfc.section.6.1 says:

The entire DATA frame payload is included in flow control, including the Pad Length and Padding fields if present.

We ignore the pad length and padding fields in our accounting.

I thought the fix was as easy as changing len(f.Data()) to f.Length in a few places, but then our accounting would be off when we return the flow control tokens on bytes read later. We'll probably want to just immediately return the flow control tokens for any padding immediately, in both the client and server.

[bradfitz]

Sent https://go-review.googlesource.com/25382 for this too. The CL should be pretty low risk for people who don't use padding (no new code paths), but pretty valuable to those who do, so I think we should probably include it in 1.7.

[gopherbot]

CL https://golang.org/cl/25382 mentions this issue.

[gopherbot]

CL https://golang.org/cl/25388 mentions this issue.