Issue #2258 - Part 3: Allow sniffing with XCTO:nosniff + empty MIME type.

This moves the checking code back to NS_SniffContent and specifically
checks requests for either:
- Empty `Content-type` header, OR
- A known JSON MIME type

If present, sniffing is allowed despite `nosniff` because in the first
case this is webmaster error (conflicting `nosniff` without MIME type
would always fail, so sniff it), and in the second case we need this
carveout or our json viewer will break.

netwerk/protocol/http/nsHttpChannel.cpp

@@ -1106,7 +1106,7 @@ ProcessXCTO(nsIURI* aURI, nsHttpResponseHead* aResponseHead, nsILoadInfo* aLoadI
         policyType == nsIContentPolicy::TYPE_SUBDOCUMENT) {
       // If the header XCTO nosniff is set for any browsing context, then
       // we set the skipContentSniffing flag on the Loadinfo. Within
-      // GetMIMETypeFromContent we then bail early and do not do any sniffing.
+      // NS_SniffContent we then bail early and do not do any sniffing.
       aLoadInfo->SetSkipContentSniffing(true);
       return NS_OK;
     }