Updates x/net/http2 to git rev 644ffc for three CLs since the last update:

http2: don't add *Response to activeRes in Transport on Headers.END_STREAM
https://golang.org/cl/19134

http2: add mechanism to send undeclared Trailers mid handler
https://golang.org/cl/19131

http2: remove unused variable
https://golang.org/cl/18936

The first in the list above is the main fix that's necessary. The
other are two are in the git history but along for the cmd/bundle
ride. The middle CL is well-tested, small (mostly comments),
non-tricky, and almost never seen (since nobody really uses Trailers).
The final CL is just deleting an unused global variable.

Fixes #14084 again (with more tests)

Change-Id: Iac51350acee9c51d32bf7779d57e9d5a5482b928
Reviewed-on: https://go-review.googlesource.com/19135
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>