fix(getGlamorClassName): don't attempt to merge class names that aren't from glamor

[emmatown]

What:

Don't attempt to merge class names that aren't from glamor

Why:

So that if you pass an emotion class name to a glamorous component an error won't be thrown. this CodeSandbox shows what happens without this fix

How:

Check if a class name is on glamor's registered cache before treating it as a glamor class name

Checklist:

• [ ] Documentation N/A
• [x] Tests
• [x] Ready to be merged
• [x] Added myself to contributors table

[kentcdodds]

I'm not sure why CI is messed up, but I'm going to guess that everything's fine...