net/http: in Client, consume small redirect bodies before making next request

net/http: in Client, consume small redirect bodies before making next request

In Go 1.2, closing a request body without reading to EOF
causes the underlying TCP connection to not be reused. This
client code following redirects was never updated when that
happened.

This was part of a previous CL but moved to its own CL at
Josh's request.  Now with test too.

LGTM=josharian
R=josharian
CC=golang-codereviews
https://golang.org/cl/70800043