Notice API: properly transform <var> tags

`<var>` tags with blank values were ending up completely wrong in the
resulting hash. 

This: (a user with a blank name)

    <var key="user">
       <var key="id">123</var>
       <var key="name"/>
    </var>

Gets parsed by hoptoad_notifier into this:

    {
       'key' => 'user',
       'var' => [{
             'key' => 'id',
             '__content__' => '123'
          },{
             'key' => 'name'
          }]
    }

Which when passed through the "rekey" method, ended up like this:

    {
       'user' => {
          'id' => '123',
          'key' => 'name' # bad and wrong
       }
    }

If there were multiple blank values, they would overwrite each-other, each using the same `'key' => ...`.


Now, after these changes, it *correctly* comes through as:

    {
       'user' => {
          'id' => '123',
          'name' => nil # much better!
       }
    }