Update url_search_params.ts

Values need to be encoded on `toString()`
Closes #7686

modules/@angular/http/src/url_search_params.ts

@@ -122,7 +122,8 @@ export class URLSearchParams {
 
   toString(): string {
     var paramsList: string[] = [];
-    this.paramsMap.forEach((values, k) => { values.forEach(v => paramsList.push(k + '=' + v)); });
+    this.paramsMap.forEach(
+        (values, k) => { values.forEach(v => paramsList.push(k + '=' + encodeURIComponent(v))); });
     return paramsList.join('&');
   }