fix firefox line item test

another scenario where browser internal rules change and the generated
DOM is slightly different than what we expect.

spec/content.spec.js

@@ -649,6 +649,7 @@ describe('Content TestCase', function () {
                 fireEvent(target, 'keydown', {
                     keyCode: MediumEditor.util.keyCode.BACKSPACE
                 });
+                this.el.innerHTML = stripLinebreak(this.el);
                 expect(this.el.innerHTML).toBe('<p>lorem ipsum</p><ul><li></li><li>lorem ipsum</li></ul>');
             });
         });
@@ -811,3 +812,7 @@ describe('Content TestCase', function () {
         });
     });
 });
+
+function stripLinebreak(element) {
+    return element.innerHTML.replace('<br>', '');
+}