The <AuModal> @size and @overflow arguments don't work together

[Windvis]

While trying out #476 I noticed that the @size argument doesn't work as expected when @overflow is set, due to missing whitespace between the classes. The issue has been around for a while, so I'm guessing these options aren't used that much 😅.

Reproduction in the current Storybook deploy: https://appuniversum.github.io/ember-appuniversum/?path=/story/components-content-aumodal--component&args=size:large;overflow:true (should show the large version, but it doesn't)