fixes #639 - withFrame() now switches back to parent frame instead of main one

@@ -2250,12 +2250,12 @@ Casper.prototype.withFrame = function withFrame(frameInfo, then) {
     } catch (e) {
         // revert to main page on error
         this.warn("Error while processing frame step: " + e);
-        this.page.switchToMainFrame();
+        this.page.switchToParentFrame();
         throw e;
     }
     return this.then(function _step() {
         // revert to main page
-        this.page.switchToMainFrame();
+        this.page.switchToParentFrame();
     });
 };