net/http: verify and test that Request.Cancel interrupts bodies too

[bradfitz]

Verify that the Transport (both http1 and http2) interrupts a body read if Request.Cancel becomes readable.

Motivation was from https://go-review.googlesource.com/#/c/16101/5..7/kubernetes/client.go

[gopherbot]

CL https://golang.org/cl/17758 mentions this issue.

[gopherbot]

CL https://golang.org/cl/17757 mentions this issue.