fix :class plain string order consistency

Author: yyx990803

src/directives/internal/class.js

@@ -72,8 +72,7 @@ function setObjectClasses (el, obj) {
 function stringToObject (value) {
   var res = {}
   var keys = value.trim().split(/\s+/)
-  var i = keys.length
-  while (i--) {
+  for (var i = 0, l = keys.length; i < l; i++) {
     res[keys[i]] = true
   }
   return res

test/unit/specs/directives/internal/class_spec.js

@@ -13,9 +13,9 @@ describe(':class', function () {
     dir.update('test')
     expect(el.className).toBe('haha test')
     dir.update('what now test')
-    expect(el.className).toBe('haha test now what')
+    expect(el.className).toBe('haha what now test')
     dir.update('ok cool')
-    expect(el.className).toBe('haha cool ok')
+    expect(el.className).toBe('haha ok cool')
     dir.update()
     expect(el.className).toBe('haha')
   })