Add custom classes in do_widget_area shortcode

The behavior of the do_widget_area shortcode doesn't seems to be aligned to the one of the do_widget (i.e. the classes specified within the class attribute in the shortcode doesn't get actually added to the widget area).

Despite the big diff (caused by trailing whitespaces I suppose) I've only modified line 39 from:

`$class = 'class="amr-widget-area ';` 

to

 `$class = "class=\"amr-widget-area {$class}";`

P.S: if I misunderstood the functionality of the whole widget area or how to accomplish what I need (i.e. add a specific class to the area just like with the single widget), please, don't hesitate to yell at me :)

P.P.S: sorry for the whitespaces :(