Fix CURLOPT_ENCODING not to be overwritten

[YoakeSakura]

This PR fixes an issue where CURLOPT_ENCODING is overwritten when decode_content is true (default is true).

Before this fix, setting only CURLOPT_ENCODING removes the Accept-Encoding header and the server response is not compressed.
After this fix, the Accept-Encoding header is sent and the server response is compressed.

Maybe this will solve that issue #2270

[Nyholm]

This needs a rebase and a test.

Also make sure CI is green

[Nyholm]

Friendly ping

[Nyholm]

Ping ping =)

[Nyholm]

@YoakeSakura will you finish this PR?

[YoakeSakura]

Thanks for the ping, sorry for the delay.
I added the test code.
MockHandler could not test the request headers that cURL adds.
So I'm testing with Server and MockHandler.

Incidentally, I think it's a good idea to add an option to automatically set the compression that cURL can support.
Now the only way to do that is to set it as a cURL option.

[Nyholm]

Fair. So if you specify $options['curl'][\CURLOPT_ENCODING] you dont want Guzzle to override it with data from the Accept-Encoding heading.

[Nyholm]

Thank you