title: dateFromString not covering valid date
body: <p>I am trying to covert a date in Mongo. We are not yet on 4.0 so I have to use dateFromString.</p>
  
  <p>My date is stored is a valid date: "2019-02-05T03:38:52.441Z"</p>
  
  <p>I can do this on my local 4.0.6 and this works great!</p>
  
  <pre><code>{ $toDate: "$eventTime" }
  </code></pre>
  
  <p>but I need to run this on an older version of Mongo.. and I changed to this:</p>
  
  <pre><code>{ $dateFromString: { dateString:  "$eventTime" }}
  
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>"$dateFromString requires that 'dateString' be a string, found: date with value 2019-02-05T03:38:52.441Z"
  </code></pre>
  
  <p>Looks like a valid date to me... any ideas? Other than upgrading Mongo :)</p>