Allow ol.Overlay positioning option to be a string

src/objectliterals.jsdoc

@@ -55,7 +55,7 @@
  * @property {Element|undefined} element The overlay element.
  * @property {ol.Coordinate|undefined} position The overlay position in map
  *     projection.
- * @property {ol.OverlayPositioning|undefined} positioning Positioning.
+ * @property {ol.OverlayPositioning|string|undefined} positioning Positioning.
  * @property {boolean|undefined} stopEvent Whether event propagation to the map
  *     viewport should be stopped. Default is `true`. If `true` the overlay is
  *     placed in the same container as that of the controls

src/ol/overlay.js

@@ -134,7 +134,8 @@ ol.Overlay = function(options) {
     this.setPosition(options.position);
   }
   if (goog.isDef(options.positioning)) {
-    this.setPositioning(options.positioning);
+    this.setPositioning(
+        /** @type {ol.OverlayPositioning} */ (options.positioning));
   }
 
 };