fixes #33 - adds presence validation support for radio btns

[jamesmk]

Inspired by @kirel's fix, this supports presence validation on radio buttons without a default.

@joecorcoran, @dannysperry, @saurabhbhatia let me know if you have any feedback, otherwise I'll merge it myself in a couple days.

thanks

[danieldocki]

My issue was fixed #57, good job :)

[saurabhbhatia]

Looks pretty good!

[danieldocki]

@jamesmk @saurabhbhatia

I was testing here and when the radio have validation inclusion, is giving as valid, when receive different values.

Example:

 validates :gender, inclusion: { in: %w(male female) }

<div class="form-group radio_buttons required user_gender">
  <label class="radio_buttons required control-label">
    <abbr title="obrigatório">*</abbr> sexo
  </label>
  <span class="radio radio-inline radio__label">
    <input class="radio_buttons required form-control" data-validate="[{&quot;kind&quot;:&quot;presence&quot;,&quot;options&quot;:{},&quot;messages&quot;:{&quot;blank&quot;:&quot;não pode ficar em branco&quot;}},{&quot;kind&quot;:&quot;inclusion&quot;,&quot;options&quot;:{&quot;in&quot;:[&quot;male&quot;,&quot;female&quot;]},&quot;messages&quot;:{&quot;inclusion&quot;:&quot;não está incluído na lista&quot;,&quot;blank&quot;:&quot;não pode ficar em branco&quot;}}]" data-klass="User" type="radio" value="malex" name="user[gender]" id="user_gender_male" style="border: 1px solid rgb(239, 72, 54);">
    <label class="collection_radio_buttons" for="user_gender_male">papai</label>
  </span>
  <span class="radio radio-inline radio__label">
    <input class="radio_buttons required form-control" data-validate="[{&quot;kind&quot;:&quot;presence&quot;,&quot;options&quot;:{},&quot;messages&quot;:{&quot;blank&quot;:&quot;não pode ficar em branco&quot;}},{&quot;kind&quot;:&quot;inclusion&quot;,&quot;options&quot;:{&quot;in&quot;:[&quot;male&quot;,&quot;female&quot;]},&quot;messages&quot;:{&quot;inclusion&quot;:&quot;não está incluído na lista&quot;,&quot;blank&quot;:&quot;não pode ficar em branco&quot;}}]" data-klass="User" type="radio" value="female" name="user[gender]" id="user_gender_female" style="">
    <label class="collection_radio_buttons" for="user_gender_female">mamãe</label>
  </span>
</div>

[jamesmk]

@danieldocki Yeah, radio btn will have all sorts of weird edge cases like this. We'd have to add a radio specific conditional to each validator. That's not a priority though, because most of the use cases don't make sense. Inclusion, for example, can be defined by the radio btn values. Adding it as a validation seems redundant.

[danieldocki]

@jamesmk make sense