fix($browser.xhr): properly delete jsonp callbacks

Author: IgorMinar

src/Browser.js

@@ -92,7 +92,7 @@ function Browser(window, document, body, XHR, $log) {
       var script = jqLite(rawDocument.createElement('script'))
           .attr({type: 'text/javascript', src: url.replace('JSON_CALLBACK', callbackId)});
       window[callbackId] = function(data){
-        window[callbackId] = undefined;
+        delete window[callbackId];
         script.remove();
         completeOutstandingRequest(callback, 200, data);
       };

test/BrowserSpecs.js

@@ -96,7 +96,7 @@ describe('browser', function(){
         fakeWindow[url[1]]('data');
         expect(callback).toHaveBeenCalled();
         expect(log).toEqual('remove();200:data;');
-        expect(typeof fakeWindow[url[1]]).toEqual('undefined');
+        expect(fakeWindow[url[1]]).toBeUndefined();
       });
     });