Update Dialog.vue (#723)

Fixes issue where dialog container does not accept class parameter

Author: christohill

src/components/Dialog.vue

@@ -3,7 +3,7 @@
  :is="$modal.context.componentName"
  name="dialog"
  height="auto"
- :classes="['vue-dialog', this.params.class]"
+ :classes="['vue-dialog', params.class]"
  :width="width"
  :shift-y="0.3"
  :adaptive="true"