net/http: fix early side effects in the ResponseWriter's ReadFrom

The ResponseWriter's ReadFrom method was causing side effects on
the output before any data was read.

Now, bail out early and do a normal copy (which does a read
before writing) when our input and output are known to not to
be the pair of types we need for sendfile.

Fixes issue 5660

R=golang-dev, rsc, nightlyone
CC=golang-dev
https://codereview.appspot.com/12632043