Timezone issue when multiple dt-*

[gRegorLove]

@kartikprabhu pointed this out with his VCP testing: When multiple date parts, the normalized timezone is being appended multiple times, resulting in the incorrect value 2014-06-01 12:30-0600-0600-0600

 <div class="h-event">
   <h1 class="p-name">Multiple date and time values</h1>

   <p> When:
     <span class="dt-start">
       <span class="value" title="June 1, 2014">2014-06-01</span>
       <span class="value" title="June 1, 3014">3014-06-01</span>
       <span class="value" title="12:30">12:30</span>
       (UTC<span class="value">-06:00</span>)
       <span class="value" title="23:00">23:00</span>
       (UTC<span class="value">+01:00</span>)
       –
       <span class="dt-end">19:30</span>
   </p>

 </div>

parsed result from current master branch:

{
    "items": [
        {
            "type": [
                "h-event"
            ],
            "properties": {
                "name": [
                    "Multiple date and time values"
                ],
                "start": [
                    "2014-06-01 12:30-0600-0600-0600"
                ],
                "end": [
                    "2014-06-01 19:30-0600"
                ]
            }
        }
    ],
    "rels": {},
    "rel-urls": {},
    "debug": {
        "package": "https://packagist.org/packages/mf2/mf2",
        "version": "dev-master",
        "note": [
            "This output was generated from the php-mf2 library available at https://github.com/indieweb/php-mf2",
            "Please file any issues with the parser at https://github.com/indieweb/php-mf2/issues"
        ]
    }
}

[aaronpk]

Fixed in #148