Respect q=0 in gzip content negotiation

[freakes]

Summary

Fix gzip middleware to respect q=0 values in the Accept-Encoding header.

Problem

According to RFC 9110, an encoding with q=0 is explicitly unacceptable. The current implementation does not respect this quality value and may enable gzip even when the client sends:

Accept-Encoding: gzip;q=0

Solution

Update the gzip negotiation logic to honor q=0 quality values and skip gzip when it is explicitly marked as unacceptable.

Testing

• added a regression test covering Accept-Encoding: gzip;q=0
• go test ./middleware