Mutating ETag response header value for response gzipped with gzip.Middleware

[pboguslawski]

According to

https://www.rfc-editor.org/rfc/rfc7232#section-2.3.3
https://bz.apache.org/bugzilla/show_bug.cgi?id=46538

ETag header values should be different for responses sent with and without content compression.

Consider allowing to generate different ETags for plain and compressed responses, maybe adding gzipETagSuffix parameter to gzip.Middleware; when suffix is not empty, append it to existing ETag response header value (inside "") when response is gzipped.

Now stripping such suffix from requests (i.e. version validation in API) will allow one to get original ETag value regardless of response encoding.