title: support for add_to_set defined in Mongoid::Atomic::Modifiers?
body: <p>Hi I'm new to mongodb,</p>
  
  <p>Need support for <code>add_to_set</code> method which add a document in a collection </p>
  
  <p>What if <code>add_to_set</code> is called twice and in both time the inserted value are equal</p>
  
  <p>for example</p>
  
  <pre><code>    someInstance.add_to_set(some_field: "hello_world")
  </code></pre>
  
  <p>if this called twice will it be an update query for mongo??</p>