title: restkit, map object with specific variabes
body: <p>I am adding an <code>attributesMappingsDictionary</code> to my <code>RKEntityMapping</code> object. This maps keys to model values, where it will get JSON and map my model object to the associated keys I told it about</p>
  
  <p>But what about circumstances where the JSON will not actually contain the proper value in its response? How do I manually set one variable to always equal a certain string that I create under certain conditions? </p>
  
  <p>How do I set <code>additionalLocalVariable: true</code>  when I am creating and caching the <code>RKEntityMapping</code> or after I get the network response?</p>