caddy-frontend: Don't compress content

Data from backend shall be returned as is, without tampering, as frontend shall be as simple as possible, so drop compressing the content.

TODOs:

• add a test that if client sends Accept-Encoding: gzip the not compressed data on the backend are not compressed by the frontend
• add a test that if client sends Accept-Encoding: gzip, and the backend compresses it, the frontend does not alter it
• merge https://lab.nexedi.com/nexedi/slapos/merge_requests/609 before