Explicitly reference className prop due to SVGAnimatedString className

[gingerchris]

As noted on MDN, the value of el.className might not be as expected when el is an SVGElement. Instead of the expected string, it can be an instance of SVGAnimatedString.

When this is the case, tests fail with the following error:

TypeError: this.classNames(...).split is not a function

This PR instead returns the contents of the element's class attribute when the element tagName is svg

[ayrton]

Can you add a test case please

[gingerchris]

I am struggling to write a test case that fails prior to this addition, which is frustrating as this is causing tests to fail in my codebase.

This codepen example demonstrates the className returning an SVGAnimatedString but when I add an svg to the Fixture in the className test it returns the className value specified in the markup.

What's the best way to get help on this? Should I create an issue? Thanks