Lowercase element type attribute

```<input type="CHECKBOX" />``` won't work properly due to not lowercasing the type first. I addressed this in my [previous commit](https://github.com/JayceTDE/value/commit/953de83fcecf1465a13c3888ef6488b6d7dcb300) to [Issue #1](https://github.com/component/value/issues/1), but that didn't end up being used.