Refactor: use global header constants instead of hardcoded strings

[StounhandJ]

Summary

Replaced hardcoded HTTP header names ("Content-Length", "Content-Encoding", "Vary", "ETag") with predefined constants throughout the gzip middleware and tests.

Motivation

In general, this is not a functional update, but rather, I noticed that they started using global constants, but only in random parts of the code. Now everything is unified.