add forwardRef to withTheme

[zepatrik]

Reasons for making this change

fixes #1405
I didn't update the docs because I would assume this is actually default behavior.

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests
  • I've updated docs if needed
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

[epicfaace]

@zepatrik ?

[epicfaace]

Suggested change

	expect(ref.current.submit).not.eql(undefined);;
	expect(ref.current.submit).not.eql(undefined);

[epicfaace]

Thanks! I was hesitant to merge until I could find out why you had to create a WrapperClassComponent for testing. It appears that it's because of the testing library we're using and it might require a refactoring of the testing framework we use in the first place, so this is good to merge!