[Input] should accommodate number and string values

[rosskevin]

The previous isControlled() was too naive and not in-line with the definition of a controlled component as defined by React. This previous implementation was the root cause of failures to trigger the dirty state on the field.

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

[rosskevin]

This works and doesn't break anything, but I need to do a bit more with isDirty to accommodate numbers.

[rosskevin]

Ok, this is good to review. In order to track the details, I've tried to be as explicit as possible with documentation and tests.

The issues were two fold:

1. isControlled check was too naive
2. isDirty check (that determines if the label is presented as placeholder or above the field) did not accommodate zero values.

[rosskevin]

BTW - I have tested it locally on docs and my own number field.

[oliviertassinari]

onChange shouldn't be taken into account. React isn't using it for native components.

[oliviertassinari]

The master branch was looking for invalid values instead of valid ones. Have you considered this option https://github.com/callemall/material-ui/blob/master/src/TextField/TextField.js#L110 ? Maybe we could make the implementation shorter/simpler.

[rosskevin]

Refactored with logic from master