Added dropifExists method

Author: thilanga

src/Jenssegers/Mongodb/Schema/Builder.php

@@ -96,6 +96,19 @@ public function create($collection, Closure $callback = null)
         }
     }
 
+    /**
+     * @inheritdoc
+     */
+    public function dropIfExists($collection)
+    {
+        if($this->hasCollection($collection)) {
+
+            return $this->drop($collection);
+        }
+
+        return false;
+    }
+
     /**
      * @inheritdoc
      */