title: How do I get RestKit to format a date when using RubyMotion
body: <p>I've setup a new RubyMotion project and have some JSON being mapped successfully to plain ruby objects. I'd like one of the attributes to be a date but I don't know how to get RestKit to do the conversion for me. </p>
  
  <p>I know how to set the date formatters but how does RestKit know which attributes to apply the date formatters to? I assume when this is done in Objective-C the data type is inferred. 
  Is there anyway to achieve this with ruby?</p>
  
  <p>Model:</p>
  
  <pre><code>def SampleModel
    attr_accessor :id, :name, :created_at
  end
  </code></pre>
  
  <p>Mapping:</p>
  
  <pre><code>def sample_mapping
    @sample_mapping ||= begin
      mapping = RKObjectMapping.mappingForClass(SampleModel)
      mapping.addAttributeMappingsFromArray(["id", "name", "created_at"])
    end
  end
  </code></pre>