Add test cases on ignoring of duplicate 'css' classes

Author: eGavr

test/differ/fixtures/second/sort-classes.html

@@ -1 +1 @@
-<input class=" cd  ab bc">
+<input class=" cd  ab bc bc">

test/unit/utils.js

@@ -29,11 +29,13 @@ describe('\'utils\'', function () {
                 { name: 'class', value: ' b c a' },
                 { name: 'blah', value: 'b c a' },
                 { name: 'class', value: '' },
+                { name: 'class', value: 'a a a' }
             ],
             output = [
                 { name: 'class', value: 'a b c' },
                 { name: 'blah', value: 'b c a' },
-                { name: 'class', value: '' }
+                { name: 'class', value: '' },
+                { name: 'class', value: 'a' }
             ];
 
         utils.sortCssClasses(input).must.be.eql(output);