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 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