feat(jqLite): don't remove a boolean attribute for .attr(attrName, '')

This change aligns jqLite with jQuery.

Ref angular#15126

BREAKING CHANGE: Before, using the `attr` method with an empty string as a value
would remove the boolean attribute. Now it sets it to its lowercase name as
was happening for every non-empty string so far. The only two values that remove
the boolean attribute are now null & false, just like in jQuery.

To migrate the code follow the example below:

Before:

elem.attr(booleanAttrName, '');

After:

elem.attr(booleanAttrName, false);

or:

elem.attr(booleanAttrName, null);

Author: mgol

src/jqLite.js

@@ -645,7 +645,7 @@ forEach({
     var lowercasedName = lowercase(name);
     if (BOOLEAN_ATTR[lowercasedName]) {
       if (isDefined(value)) {
-        if (value) {
+        if (value !== false && value !== null) {
           element.setAttribute(name, name);
         } else {
           element.removeAttribute(name);

test/jqLiteSpec.js

@@ -733,6 +733,24 @@ describe('jqLite', function() {
       elm.attr('attribute', '');
       expect(elm[0].getAttribute('attribute')).toBe('');
     });
+
+    it('should remove the boolean attribute for a false value', function() {
+      var elm = jqLite('<select multiple>');
+      elm.attr('multiple', false);
+      expect(elm[0].hasAttribute('multiple')).toBe(false);
+    });
+
+    it('should remove the boolean attribute for a null value', function() {
+      var elm = jqLite('<select multiple>');
+      elm.attr('multiple', null);
+      expect(elm[0].hasAttribute('multiple')).toBe(false);
+    });
+
+    it('should not remove the boolean attribute for an empty string as a value', function() {
+      var elm = jqLite('<select multiple>');
+      elm.attr('multiple', '');
+      expect(elm[0].getAttribute('multiple')).toBe('multiple');
+    });
   });