Improved Flagging for Class Names

[jonobr1]

Previously when setting Two.Element.className multiple times the developer could overwrite the flag for sending updates to be rendered like so:

var path = new Two.Path();
path.className = 'foo';
path.className = 'foo';

Because of the way the setter was set up the flag would be updated because the first time it was called it would be set to true. But, the second invocation would show that 'foo' === 'foo' and thus would turn the flag off. This PR fixes that.