title: Time stopped when I use mongoose with default: new Date()
body: <p>My node version is 6.11.4, and mongoose version is 4.12.4.<br/>
  I only show the <code>createTime</code> field in <code>users</code> schema here.</p>
  
  <pre><code>createTime: {
      type: Date,
      default: new Date()
  }
  </code></pre>
  
  <p>When I called a <code>userCreate(user)</code> method to create a user document, and an exception appeared like this.</p>
  
  <pre><code>userCreate(user);
  console.log(undefined.name);
  </code></pre>
  
  <p>This document was created and the createTime field was correct.
  However, when I run this two lines again, the new document's createTime was stuck as the last document's createTime.<br/>
  I don't know if this is a javascript feature or I used mongoose in a wrong way.<br/>
  Anyone could help me?</p>