Consistent camelBacked method casing

Many projects use classnames like JsonView (for JSON view output), RssView, XmlView etc.
Same for methods regarding those abbrevations.

I recommend using those for Carbon, as well, for readability.

    ->toRSSString();

really looks strange and it is difficult to count the amount of S in there on first glance.
It is way better to use

    ->toRssString();

here, as well.

Since casing is case insensitive, this change is 100% BC.

Follow up on https://github.com/cakephp/cakephp/pull/4710