souphttpsrc: Always use the content decoder but set `Accept-Encoding:...

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 gstreamer/gst-plugins-good#833

Part-of: <gstreamer/gst-plugins-good!1054>
31 jobs for !1054 with backport-mr-1053-into-1.18-souphttpsrc-decompress-always in 24 minutes and 26 seconds (queued for 7 seconds)
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