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.

image/imgLoader.cpp

@@ -2489,13 +2489,6 @@ imgLoader::GetMIMETypeFromContent(nsIRequest* aRequest,
                                   uint32_t aLength,
                                   nsACString& aContentType)
 {
-  nsCOMPtr<nsIChannel> channel(do_QueryInterface(aRequest));
-  if (channel) {
-    nsCOMPtr<nsILoadInfo> loadInfo = channel->GetLoadInfo();
-    if (loadInfo->GetSkipContentSniffing()) {
-      return NS_ERROR_NOT_AVAILABLE;
-    }
-  }
   return GetMimeTypeFromContent((const char*)aContents, aLength, aContentType);
 }