compression: GET and HEAD headers do not match

*Title*: Content-Encoding header is not added to HEAD responses.

*Description*:
Following header is not present in HEAD responses:
```content-encoding: gzip```

[RFC7231](https://tools.ietf.org/html/rfc7231#section-4.3.2) specifies the following:
> The server SHOULD send the same
> header fields in response to a HEAD request as it would have sent if
> the request had been a GET, except that the payload header fields
> (Section 3.3) MAY be omitted.

(NB! Content-Encoding is not in the list of payload-specific headers.)

Therefore this may cause issues with clients that follow RFC and expect HEAD and GET headers to match.

cc: @rojkov @antoniovicente