net/http: update bundled http2
Updates http2 to x/net/http2 git rev
5f9ae10 for:
http2: terminate await request cancel goroutine on conn close
https://golang.org/cl/108415
http2: don't sniff Content-type in Server when X-Content-Type-Options:nosniff
https://golang.org/cl/107295
http2, http/httpguts: move ValidTrailerHeader to new common package http/httpguts
https://golang.org/cl/104042
all: remove "the" duplications
https://golang.org/cl/94975
http2: use RFC 723x as normative reference in docs
https://golang.org/cl/94555
all: use HTTPS for iana.org links
https://golang.org/cl/89415
Fixes #24795
Fixes #24776
Updates #23908
Fixes #21974
Change-Id: I7985617a7dde56cc5ed8670d73b26f8307be83d6
Reviewed-on: https://go-review.googlesource.com/111655
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>