Content-Encoding: gzip header should not be sent when HTTP 204

[mtojek]

Hi,

nginx warns about empty responses (HTTP 204) with a Content-Encoding header set (based on user request header Accept-Encoding).

I propose to refactor a bit gzipResponseWriter to be sending mentioned header if only HTTP response body is not empty.

Will it be possible, what do you think @vishr ?

Best regards,
Marcin

[vishr]

Not a problem, just do a quick rfc check and send a PR or even I can do it.

[Taik]

Sorry I don't have time to send a PR, but according to https://tools.ietf.org/html/rfc7230#section-3.3.1:

   A server MUST NOT send a Transfer-Encoding header field in any
   response with a status code of 1xx (Informational) or 204 (No
   Content).  A server MUST NOT send a Transfer-Encoding header field in
   any 2xx (Successful) response to a CONNECT request (Section 4.3.6 of
   [RFC7231]).

So I think the easiest case is to just follow what @mtojek suggested.

[vishr]

@mtojek @Taik Looks like this is already fixed here https://github.com/labstack/echo/blob/master/middleware/compress.go#L62. Can you guys double check?