Fixes .when attribute

From documentation it seems that `when` is a attribute of `<input>`. This way you can configure a different event for each input:

    $('#awesomeForm').isHappy({
      fields: {
        // reference the field you're talking about, probably by `id`
        // but you could certainly do $('[name=name]') as well.
        '#yourName': {
          required: true,
          message: 'Might we inquire your name',
          when: 'keyup'
        }
      },
    });

Previously the attribute was set over the `fields` object, if you set this value it will change the event for all fields.