Warn about br handling if brotlipy is not installed

Currently, the HTTP compression middleware sets the Accept-Encoding header to gzip,deflate or gzip,deflate,br depending on whether or not brotlipy is installed.

However, if the user manually sets the Accept-Encoding header to a value containing br, and does not have brotlipy installed, the server can send a response compressed with br and Scrapy will silently fail to decompress it, so the response content will not be readable.

I think we should improve a bit the handling of this scenario.

If the user sets br manually on the Accept-Encoding header, I think we should:

• Log a warning if brotlipy is not installed.
• Have the middleware cause an error if the server sends the response br-encoded, and brotlipy is not installed.
• Log an information message the first time that the user sends a response with a manually-set Accept-Encoding header which is identical to the one that the HTTP compression middleware would define. It should not be a warning because the user does not need to do anything about it. It should only be displayed once because there is a chance the user is doing it on purpose (e.g. if the default headers define a different value but the user wishes to override that for some requests with the default value.

The 3rd bullet point may not be that useful, but I think the 1st and 2nd are.

[Gallaecio]

It’s quite a good start, but it would need some adjustments to get merged. It should definitely make things easier for the next person that comes around.

[wRAR]

Alternatively: #4698