feat(rxForm): Fixes required attribute to checkboxes in rxOptionTableForm

Resolves #377 .
To save on space, I'd move the initial array to the top of the file and just copy it down for each test:
```
describe('rxFormOptionTable (Checkbox)', function () {
    var scope, compile, rootScope;

    var tableDataTemplate = [
    {
        'name': 'Item 1'
    }, 
    {
        'name': 'Item 2'
    }];

    beforeEach(function () {
        ...
    });

    it('should validate if there is an empty form but no required flag', function () {
        var checkboxFormTemplate =
            '<rx-form-option-table data="tableData" columns="tableColumns" ' +
            'type="checkbox" model="myModel" field-id="optionTable2"></rx-form-option-table>';

        var checkScope = rootScope.$new();
        checkScope.tableData = _.clone(tableDataTemplate);
        checkScope.myModel = [true, false];

        var checkTable = helpers.createDirective(checkboxFormTemplate, compile, checkScope);
        var checkTableScope = checkTable.isolateScope();
        expect(checkTableScope.checkRequired()).to.be.false;
    });

};
```