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>