net/http: RoundTrip unexpectedly changes Request [1.14 backport]

[gopherbot]

@dmitshur requested issue #39533 to be considered for backport to the next 1.14 minor release.

@gopherbot Please backport to Go 1.14.

This is a fixup to a previous fix (CL 242117) for an approved backport issue #39279. We can't submit that CL without this one. This needs to be backported to Go 1.14 only, since the fix is already included in Go 1.1​5.

[gopherbot]

Change https://golang.org/cl/249880 mentions this issue: [release-branch.go1.14] net/http: make Transport.RoundTrip preserve Requests

[cagedmantis]

Approved because this is a critical problem without a workaround.

[gopherbot]

Closed by merging fae8e09 to release-branch.go1.14.