fix(grid-stack): allow custom class attribute

Author: jkusa

addon/components/grid-stack.hbs

@@ -5,6 +5,7 @@
   {{did-insert this.setup}}
   {{did-update this.update @options}}
   {{will-destroy this.willDestroyNode}}
+  ...attributes
 >
   {{yield this}}
 </div>

tests/integration/components/grid-stack-test.js

@@ -14,19 +14,19 @@ module('Integration | Component | grid stack', function (hooks) {
     assert.expect(2);
 
     await render(hbs`
-      <GridStack>
+      <GridStack class="test-grid">
         <div class="grid-stack-item" gs-x="0" gs-y="0" gs-w="1" gs-h="1">
           <div class="grid-stack-item-content">My Widget</div>
         </div>
       </GridStack>
     `);
 
     assert
-      .dom('.grid-stack .grid-stack-item')
+      .dom('.test-grid.grid-stack .grid-stack-item')
       .hasClass('ui-draggable', 'Dom elements with grid-stack-item class are initialized by gridstack');
 
     assert
-      .dom('.grid-stack .grid-stack-item')
+      .dom('.test-grid.grid-stack .grid-stack-item')
       .hasClass('ui-resizable', 'Dom elements with grid-stack-item class are initialized by gridstack');
   });