Edit file only if necessary 

I've added the following line to my `.vimrc` which runs `JsBeautify` every time I save a file:

``` viml
au BufWritePre *.{js} :call JsBeautify()
```

Most of the time `JsBeautify` doesn't have any effect but replaces the whole content of the file leaving an extra step in the undo history. It should only edit the content of a file if there is a change.
