bug of function removeClass

 vue.js(for dev) ,  Line 817, removeClass funciton,`else` branch

```
             //assume el.className='red yellow green'
             el.className = (' ' + 'red yellow green' + ' ')
            .replace(' ' + 'yellow' + ' ', '')
            .trim()
            // will be 'redgreen',not 'red green' 
```
