colorWithOpacity tests and return consistency fix

[fleur]

Hi. Added a few more tests. I didn't include these in my last PR because the way the cacheing and multiplying the alpha is done initially confused me.

Also, the way colorWithOpacity() was written, it was an identity function if no opacity is passed in, meaning it returned a Color, but if opacity was provided, an array is returned. Strictly speaking it doesn't affect functionality, but it took me awhile to convince myself it didn't matter, so for code maintenance purposes, it's a benefit. I hope you'll agree!

[ahocevar]

Thanks for the additional tests. I do not agree with the change to the colorWithOpacity function though, because it adds a performance penalty. Instead, maybe just rename the function or add a @return annotation with type to avoid future confusion.