Fix deprecation warning bug, closes #2876

lib/associations/belongs-to-many.js

@@ -42,7 +42,7 @@ module.exports = (function() {
       this.through = true;
     }
 
-    if (this.through) {
+    if (this.through === true) {
       deprecated('belongsToMany: not defining through or defining through as true has been deprecated, please provide a string or a model');
     }