Support "gzip" aka "x-gzip" as a transfer-encoding for
requests and responses as per RFC 7230 Section 3.3.1.

"gzip" and "x-gzip" are equivalents as requested by
RFC 7230 Section 4.2.3.

Transfer-Encoding is an on-fly property of the body
that can be applied by proxies, other servers and basically
any intermediary to transport the content e.g. across data centers
or backends/machine to machine that need compression.

For this change, "gzip" is both explicitly and implicitly combined
with transfer-encoding "chunked" in an ordering such as:

    Transfer-Encoding: gzip, chunked

and NOT

    Transfer-Encoding: chunked, gzip

Obviously the latter form is counter-intuitive for streaming.
Thus "chunked" is the last value to appear in that transfer-encoding header,
if explicitly included.

When parsing the response, the chunked body is concatenated as "chunked" does,
before finally being decompressed as "gzip".

A chunked and compressed body would typically look like this:

<LENGTH_1>\r\n<CHUNK_1_GZIPPED_BODY>\r\n<LENGTH_2>\r\n<CHUNK_2_GZIPPED_BODY>\0\r\n

which when being processed we would contentate

    <FULL_BODY>  := <CHUNK_1_GZIPPED_BODY> + <CHUNK_2_GZIPPED_BODY> + ...

and then finally gunzip it
    <FINAL_BODY> := gunzip(<FULL_BODY>)

If a "chunked" transfer-encoding is NOT applied but "gzip" is applied,
we implicitly assume that they requested using "chunked" at the end.
This is as per the recommendation of RFC 3.3.1. which explicitly says
that for:

* Request:
"  If any transfer coding
   other than chunked is applied to a request payload body, the sender
   MUST apply chunked as the final transfer coding to ensure that the
   message is properly framed."

* Response:
"  If any transfer coding other than
   chunked is applied to a response payload body, the sender MUST either
   apply chunked as the final transfer coding or terminate the message
   by closing the connection."

RELNOTE=yes

Fixes #29162

Change-Id: Icb8b8b838cf4119705605b29725cabb1fe258491
Reviewed-on: https://go-review.googlesource.com/c/go/+/166517
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>