Fix input/textarea `value` of number 0 and `false`

Previously, setting textarea `value` to number 0 is treated as if `value` wasn't set at all (thus the textarea is cleared from 0 to '' upon `onChange`). `false` also renders as `"false"` instead of `""` for both `defaultValue` and `value`, on textarea _and_ input.
Merge pull request #274 from chenglou/textarea-patch

fix textarea `value` of number 0