fields.number() doesn't enforce number.

When I use `fields.number()`, it validates even if the field is not a number.

``` js
import forms, { fields, validators } from 'forms';

const nestedForm = forms.create({
age: fields.number({ required: validators.required('age') }),
});

const handlers = {
success(form) {
console.log('success');
},

other(form) {
console.log('fail');
},
};

nestedForm.handle({ age: 'this is a text' }, handlers);
// console prints "success"
```