[cssom][css-variable] Methods of CSSStyleDeclaration interface doesn't handle custom properties properly

[upsuper]

In all methods which take a property parameter, there is a very early step says:

Let property be property converted to ASCII lowercase.

However, name of custom properties is case-sensitive, so this step would mess up custom properties.

In addition to that, some of the methods also have:

If property is not a case-sensitive match for a supported CSS property, terminate this algorithm.

This would make the method not usable for custom properties.

I think the solution is to recognize custom properties in an early step, and only apply the above steps to non-custom properties.

[upsuper]

Looks like it is a dup of #580.