Bugfix: Medium URL question mark wasn't being added properly

It was adding the question-mark only if ? was found at position 0
(i.e. at the beginning) instead of at position -1 (i.e. not present
at all).

seance.js

@@ -340,7 +340,7 @@ class Seance {
 
       // intelligently add ?json attribute
       if (mediumUrl.indexOf('format=json') == -1) {
-        if (mediumUrl.indexOf('?') == 0) {
+        if (mediumUrl.indexOf('?') == -1) {
           mediumUrl = `${mediumUrl}?format=json`
         } else {
           mediumUrl = `${mediumUrl}&format=json`