fix compress middleware: exact match and respect q=0 in Accept-Encoding

[Emil-Ka]

Fixes #1069

matchAcceptEncoding used strings.Contains which caused two bugs:

1. Substring false positives — Accept-Encoding: br incorrectly matched encoding b, bgzip matched gzip
2. q=0 ignored — per RFC 9110 §12.5.3, q=0 means the encoding is explicitly not acceptable, but strings.Contains("gzip;q=0", "gzip") returned true

Fix: parse the encoding token properly — split on ;, trim whitespace, exact-match the name, reject when q=0.

Added unit tests covering all edge cases.