souphttpsrc: Always use the content decoder but set `Accept-Encoding: identity` if no compression should be used

Some servers respond with gzip-encoded responses regardless of whether
the request allowed it to be used in the response. By always having the
content decoder enabled, these invalid responses can be decoded
correctly while for well-behaving servers the `compress` property
selects between allowing compressed responses or not.

Fixes #833

Part-of: <!1053>