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
       }
    }

Now, after these changes, it correctly comes through as:
    {
       'user' => {
          'id' => '123',
          'name' => nil # much better!
       }
    }
Merge pull request #442 from iFixit/notice-xml-parsing-fix-blank

Notice API: properly transform <var> tags