mapic: Fix multistream target URL ? handling (#1300)

It turns out that Mist has a special parsing of these push URLs which
is not a regular URL parsing. It removes anything after the last `?`
in the URL and uses that as its config. If there are multiple `?` in
the URL (e.g. query-string already existed in target URL), it will
strip only (from) the last one and keep the previous.

So instead of merging Mist's qs with the existing ones, we actually
need to create a separate `?` section for Mist. This guarantees both
that Mist gets its config properly, as well as that it preserves the
URL to call the target service.

mapic/mistapiconnector_app.go

@@ -748,16 +748,13 @@ func (mc *mac) getPushUrl(stream *api.Stream, targetRef *api.MultistreamTargetRe
 		}
 		videoSelector = fmt.Sprintf("~%dx%d", prof.Width, prof.Height)
 	}
-	join := "?"
-	if strings.Contains(target.URL, "?") {
-		join = "&"
-	}
 	audioSelector := "maxbps"
 	if targetRef.VideoOnly {
 		audioSelector = "silent"
 	}
-	// Inject ?video=~widthxheight to send the correct rendition
-	return target, fmt.Sprintf("%s%svideo=%s&audio=%s", target.URL, join, videoSelector, audioSelector), nil
+	// Inject ?video=~widthxheight to send the correct rendition. Notice that we don't care if there is already a
+	// query-string in the URL since Mist will always strip from the last `?` in the push URL for its configs.
+	return target, fmt.Sprintf("%s?video=%s&audio=%s", target.URL, videoSelector, audioSelector), nil
 }
 
 func (mc *mac) getStreamInfoLogged(playbackID string) (*streamInfo, bool) {