upstream: Forward Content-Type from upstream if we don't already have…

… one

For example, our RESTful endpoints perform content negotiation and use
the result to determine Content-Type (and also what to Accept in the
request to upstream), but our Charon endpoints do not set Content-Type
at all.  Set it based on the upstream response in that situation.

I noticed this bug (while investigating another issue¹) because the
automatic compression middleware wasn't applying to the
/charon/getDataset endpoint due to the lack of Content-Type.

¹ <#832>

src/upstream.js

@@ -234,6 +234,11 @@ async function proxyResponseBodyFromUpstream(req, res, upstreamReq) {
      * client can decode the body itself.
      */
     ...(!upstreamReq.compress ? ["Content-Encoding"] : []),
+
+    /* Forward Content-Type if our response doesn't already have a preferred
+     * type set (e.g. from prior content negotiation).
+     */
+    ...(!res.get("Content-Type") ? ["Content-Type"] : []),
   ];
 
   res.set(copyHeaders(upstreamRes.headers, forwardedUpstreamResHeaders));