The Content-Length header would unconditionally be included when the
proxy property was set. This would result in requests with both
Content-Length and Transfer-Encoding header. Now we rely on the
use-content-length property in the proxy case aswell. This also makes
sure that Content-Type is set correctly, since before that would be
skipped if proxy was used.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7935>
e9d32abf98