Patch React 15 warnings

[ifunk]

Fixes this warning (#150) when value for input is undefined or null by using an empty string for value when required.

A component is changing a controlled input of type text to be uncontrolled.
Input elements should not switch from controlled to uncontrolled (or vice versa).
Decide between using a controlled or uncontrolled input element for the
lifetime of the component. More info: https://fb.me/react-controlled-components

Fixes this warning by removing known keys from spreaded props.

Warning: Unknown props `stylesheet`, `formValue` on <form> tag.
Remove these props from the element. For details, see https://fb.me/react-unknown-prop

[ifunk]

I should note preventing spreading of props might be a breaking change for people who are passing through a component for Fieldset instead of a dom string, but they should be able to refactor their code to use Fieldset directly inside their component instead.

[digitalkaoz]

any chance this gets merged?

[akomm]

Would appreciate if you could merge it

[nightwolfz]

Pretty please?