x/net/http2: Client doesn't send body until ExpectContinueTimeout expires [1.17 backport]

[gopherbot]

@neild requested issue #49677 to be considered for backport to the next 1.17 minor release.

@gopherbot Please open backport issues for 1.16 and 1.17. This is a regression.

[gopherbot]

Change https://golang.org/cl/368475 mentions this issue: [internal-branch.go1.17-vendor] net/http2: Fix handling of expect continue

[gopherbot]

Change https://golang.org/cl/368574 mentions this issue: [release-branch.go1.17] net/http: update bundled golang.org/x/net/http2

[gopherbot]

Closed by merging 0177913 to release-branch.go1.17.