Fix dt- parsing adding "T" separator

[gRegorLove]

Datetime properties should be parsed as authored. If the "T" datetime separator isn't in the HTML, don't add it.

<div class="h-entry">
  <span class="dt-start">
    <time class="value" datetime="2009-06-26">26 July</time>, from
    <time class="value">19:00:00-08:00</time> 
  </span>
</div>

Expected:

{
    "items": [
        {
            "type": [
                "h-entry"
            ],
            "properties": {
                "start": [
                    "2009-06-26 19:00:00-08:00"
                ],
                "name": [
                    "26 July, from\r\n    19:00:00-08:00"
                ]
            }
        }
    ],
    "rels": {}
}

http://microformats.org/wiki/value-class-pattern#Date_and_time_parsing
http://microformats.org/wiki/microformats2-parsing-issues#Standard_datetime_format