net/http, strings, bytes: fix http race, revert part of Reader behavior change

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

src/pkg/strings/strings_test.go

@@ -862,7 +862,7 @@ var UnreadRuneErrorTests = []struct {
 	name string
 	f    func(*Reader)
 }{
-	{"Read", func(r *Reader) { r.Read([]byte{}) }},
+	{"Read", func(r *Reader) { r.Read([]byte{0}) }},
 	{"ReadByte", func(r *Reader) { r.ReadByte() }},
 	{"UnreadRune", func(r *Reader) { r.UnreadRune() }},
 	{"Seek", func(r *Reader) { r.Seek(0, 1) }},