http: ensure correct header handling

[StounhandJ]

Summary

During processing by middleware, the gzip headers from the original response were removed, causing the loss of their original integrity.

Changes

• Introduced headerWrited flag to prevent duplicate Content-Encoding/Vary headers.
• Removed obsolete removeGzipHeaders method.
• Updated tests to check correct behavior of Vary and ETag headers.

Motivation

In general, this is not a functional update, but is aimed at improving the readability and correctness of working with HTTP headers, including ETag and Vary, with gzip compression.

Impact

• No functional changes to response compression.
• Headers are now handled more consistently.
• Tests updated to validate new header logic.