title: Manually setting timestamp values in bookshelf.js
body: <p>I have a table set up to have timestamps and bookshelf configured to use them.  Normally everything happens as it should and bookshelf takes care of the timestamps, but I have a case where I want to specify them, but when I try to do that the values are ignored and the current date is used.</p>
  
  <p>I've tried to simplify my use case down to the essentials:</p>
  
  <pre><code>var Author = Bookshelf.Model.extend({
    tableName: 'authors',
    hasTimestamps: ['created_at', 'updated_at'],
  
    bookAuthors: function(){
      return this.hasMany(require('.book_authors'));
    },
  
    associateBookWithAuthor(bookId,timestamp) {
      return self.related('bookAuthors').create({
        book_id: bookId,
        updated_at: timestamp, // ignored by bookshelf
        created_at: timestamp  // also ignored
      })
    }
  }
  </code></pre>
  
  <p>I still want to keep the <code>hasTimestamps</code> configured for regular use cases.   Is it possible to get Bookshelf to let me override the timestamps behavior?</p>