Update helper casing

[joeldrapper]

Rails appears to have recently corrected the casing of some of their helper methods, e.g. text_area -> textarea and check_box -> checkbox. We should update the adapters to match, while continuing to support the old casing.

[joeldrapper]

Even though I would argue it should be textarea in lowercase, I wonder if the uppercase version should continue to be TextArea rather than Textarea. In the DOM API, it’s TextArea. Checkbox however is definitely one word.