allow empty/falsy values in v-bind:class array syntax

Author: yyx990803

src/directives/internal/class.js

@@ -32,7 +32,9 @@ module.exports = {
   handleArray: function (value) {
     this.cleanup(value)
     for (var i = 0, l = value.length; i < l; i++) {
-      addClass(this.el, value[i])
+      if (value[i]) {
+        addClass(this.el, value[i])
+      }
     }
     this.prevKeys = value
   },

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

@@ -47,6 +47,8 @@ if (_.inBrowser) {
       expect(el.className).toBe('a c d')
       dir.update()
       expect(el.className).toBe('a')
+      dir.update(['e', ''])
+      expect(el.className).toBe('a e')
     })
 
   })