I fixed this data race regression in two ways: in net/http itself, and also
partially reverting the change from https://golang.org/cl/77580046 .
Previously a Read from a strings.Reader or bytes.Reader returning 0 bytes
would not be a memory write. After 77580046 it was. This reverts that back
in case others depended on that. Also adds tests.

Fixes #7856

LGTM=ruiu, iant
R=iant, ruiu
CC=golang-codereviews, gri
https://golang.org/cl/94740044