Conditional compression based on request Content-Type

[ldez]

What does this PR do?

Allow to exclude some Content-Type of the compression.

Motivation

Fixes #2576
Closes #5120

More

• Added/updated tests
• Added/updated documentation

[jbdoumenjou]

LGTM

[dduportal]

It does not work on my test:

I got the following error message multiple times

mime: no media type" middlewareName=jenkins-compress@docker middlewareType=Compress

For reproduction: I used the following example:

version: '3'

services:
  proxy:
    # Built with "make build-image"
    # from https://github.com/containous/traefik/pull/5721/commits/d6a2d32b5c306141312811dac74d5a9da5a53c61
    image: 76025322bc49 
    command:
      - --providers.docker
      - --entrypoints.web.address=:80
      - --api.insecure=true
    ports:
      - "80:80"
      - "8080:8080"
    labels:
      - traefik.enable=false
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro

  cloudbees-jenkins:
    image: cloudbees/jenkins-enterprise:latest
    restart: unless-stopped
    labels:
      - traefik.http.routers.jenkins.rule=Host(`jenkins.localhost`)
      - traefik.http.services.jenkins.loadbalancer.server.port=8080
      - traefik.http.middlewares.jenkins-compress.compress=true
      - traefik.http.middlewares.jenkins-compress.compress.excludedcontenttypes=application/javascript
      - traefik.http.routers.jenkins.middlewares=jenkins-compress

I expect any requests to application/json file to NOT be compressed ,
but it is still gzipped:

Di I do something wrong?

[ldez]

application/javascript != application/json

[dduportal]

After an exchange:

• The "application/javascript != application/json" is an error on my previous GH message, but the configuration was always application/javascript.
• Error on my side is the expectation: Expecting the Content-Type Header of responses from backend to be the switch, while it the incoming request. Fix: Even though it is correctly written in the proposed documentation, we added a 2nd mention to the "incoming request" to be sure (a warning would have been heavy).
• The message about empty content-type is expected as my requests did not have any, but we decided to move it as "debug" level to avoid flooding end-users.

[dduportal]

LGTM 👍

[dtomcej]

LGTM