title: Change json keys in JMSSerializerBundle
body: <p>Suppose I have an entity with a single field: 'userName' whos corresponding column name in the table is 'user_name'. When I serialize object of that entity, it returns a json with column name of the userName field like this:</p>
  
  <pre><code>{"user_name": "davit"}
  </code></pre>
  
  <p>but what I want is for jms serializer to use entity field name (userName) as the json key:</p>
  
  <pre><code>{"userName": "davit"}    
  </code></pre>
  
  <p>Is this doable? </p>
  
  <p>I read a lot of articles beginning with the documentation, but failed to find a reference to this...</p>
  
  <p>Thanks a lot for help</p>