net/http: fix handling of HEAD in ReadResponse and (*http.Response).Write

The test suite for ReadResponse was not checking the error return on the io.Copy
on the body. This was masking two errors: the handling of chunked responses to
HEAD requests and the handling of Content-Length > 0 to HEAD.

The former manifested itself as an 'unexpected EOF' when doing the io.Copy
because a chunked reader was assigned but there were no chunks to read. The
latter cause (*http.Response).Write to report an error on HEAD requests
because it saw a Content-Length > 0 and expected a body.

There was also a missing \r\n in one chunked test that meant that the chunked
encoding was malformed. This does not appear to have been intentional.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7407046

Committer: Brad Fitzpatrick <bradfitz@golang.org>