Allow non-string values to be used as ownValue

[wsmd]

Fixes #14

<input {...input.checkbox('options', 'value')} />;
<input {...input.checkbox('options', true)} />;
<input {...input.checkbox('options', 123)} />;
<input {...input.checkbox('options', ['a', 'b'])} />;

{
  "values": {
    "options": [
      "value",
      "123",
      "a,b",
      "true"
    ]
  }
}

<input {...input.radio('option', '1')} />;
<input {...input.radio('option', 1)} />;
<input {...input.radio('option', ['1'])} />;

{
  "values": {
    "option": "1"
  }
}

[coveralls]

Pull Request Test Coverage Report for Build 135

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 122:	0.0%
Covered Lines:	84
Relevant Lines:	84

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💛 - Coveralls

[wsmd]

Per spec:

the user agent must set the value of the element to the value of the value content attribute, if there is one, or the empty string otherwise

Therefore, if ownValue isn't specified by the user, it should return an empty string. Previously, it was returning undefined. This should not result in any breaking changes.