feat(jqLite): remove the attribute for .attr(attribute, null)

This change aligns jqLite with jQuery.

Also, the extra `2` second parameter to `setAttribute` has been removed;
it was only needed for IE<9 and latest jQuery doesn't pass it either.

Ref #15126

BREAKING CHANGE: Invoking `elem.attr(attributeName, null)` would set the
`attributeName` atribute value to a string `"null"`, now it removes the
attribute instead.

To migrate the code follow the example below:

Before:

elem.attr(attributeName, null);

After:

elem.attr(attributeName, "null");

src/jqLite.js

@@ -654,11 +654,13 @@ forEach({
         return element.getAttribute(name) != null ? lowercasedName : undefined;
       }
     } else if (isDefined(value)) {
-      element.setAttribute(name, value);
+      if (value === null) {
+        element.removeAttribute(name);
+      } else {
+        element.setAttribute(name, value);
+      }
     } else if (element.getAttribute) {
-      // the extra argument "2" is to get the right thing for a.href in IE, see jQuery code
-      // some elements (e.g. Document) don't have get attribute, so return undefined
-      var ret = element.getAttribute(name, 2);
+      var ret = element.getAttribute(name);
       // normalize non-existing attributes to undefined (as jQuery)
       return ret === null ? undefined : ret;
     }

test/jqLiteSpec.js

@@ -721,6 +721,18 @@ describe('jqLite', function() {
       expect(comment.attr('some-attribute','somevalue')).toEqual(comment);
       expect(comment.attr('some-attribute')).toBeUndefined();
     });
+
+    it('should remove the attribute for a null value', function() {
+      var elm = jqLite('<div attribute="value">a</div>');
+      elm.attr('attribute', null);
+      expect(elm[0].hasAttribute('attribute')).toBe(false);
+    });
+
+    it('should not remove the attribute for an empty string as a value', function() {
+      var elm = jqLite('<div attribute="value">a</div>');
+      elm.attr('attribute', '');
+      expect(elm[0].getAttribute('attribute')).toBe('');
+    });
   });