Reaffirm support for BelongsToMany relations

This temporarily skirts around the issue described in the following issue: #1293

Author: benargo

src/Jenssegers/Mongodb/Helpers/QueriesRelationships.php

@@ -112,7 +112,11 @@ protected function getRelatedConstraintKey($relation)
             return $relation->getForeignKey();
         }
 
-        throw new \Exception(class_basename($relation) . ' Is Not supported for hybrid query constraints!');
+        if ($relation instanceof BelongsToMany && ! $this->isAcrossConnections($relation)) {
+            return $this->model->getKeyName();
+        }
+
+        throw new \Exception(class_basename($relation) . ' is not supported for hybrid query constraints.');
     }
 
     /**