http: ignore Transfer-Encoding on HEAD responses

http: ignore Transfer-Encoding on HEAD responses

Amazon S3 sends Transfer-Encoding "chunked"
on its 404 responses to HEAD requests for
missing objects.

We weren't ignoring the Transfer-Encoding
and were thus interpretting the subsequent
response headers as a chunk header from the
previous responses body (but a HEAD response
can't have a body)

R=rsc, adg
CC=golang-dev
https://golang.org/cl/4346050