net/http, x/net/http2: net/http's TestNoSniffExpectRequestBody_h2 stops passing after x/net/http2 change in CL 253259 [1.14 backport]

[gopherbot]

@answer1991 requested issue #42498 to be considered for backport to the next 1.14 minor release.

@gopherbot please backport, as we backport https://golang.org/cl/253259 before and which will cause this issue.

[dmitshur]

Closing for the same reason as in #43139.