MM-15835: correct errors and content types for oauth api calls (#10999)

* MM-15835: correct errors and content types for oauth api calls

* Addressing PR review comments

web/handlers.go

@@ -195,7 +195,7 @@ func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 			c.Err.IsOAuth = false
 		}
 
-		if IsApiCall(c.App, r) || IsWebhookCall(c.App, r) || len(r.Header.Get("X-Mobile-App")) > 0 {
+		if IsApiCall(c.App, r) || IsWebhookCall(c.App, r) || IsOAuthApiCall(c.App, r) || len(r.Header.Get("X-Mobile-App")) > 0 {
 			w.WriteHeader(c.Err.StatusCode)
 			w.Write([]byte(c.Err.ToJson()))
 		} else {