net/http: fix Content-Length/Transfer-Encoding on HEAD requests

net/http: fix Content-Length/Transfer-Encoding on HEAD requests

net/http currently assumes that the response to a HEAD request
    will always have a Content-Length header. This is incorrect.

RFC2616 says: "The HEAD method is identical to GET except that
the server MUST NOT return a message-body in the response. The
metainformation contained in the HTTP headers in response to a
HEAD request SHOULD be identical to the information sent in
response to a GET request. This method can be used for
obtaining metainformation about the entity implied by the
request without transferring the entity-body itself. This
method is often used for testing hypertext links for validity,
accessibility, and recent modification."

This means that three cases are possible: a Content-Length
header, a Transfer-Encoding header or neither. In the wild the
following sites exhibit these behaviours (curl -I):

HEAD on http://www.google.co.uk/ has Transfer-Encoding: chunked
HEAD on http://www.bbc.co.uk/    has Content-Length: 45247
HEAD on http://edition.cnn.com/  has neither header

This patch does not remove the ErrMissingContentLength error
for compatibility reasons, but it is no longer used.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7182045