title: Any way to specify a non-returned value for an RKEntityMapping?
body: <p>Because I am working with a non-KVC REST service I have a need to designate a <code>bool</code> attribute <code>isSpecial</code> by the request URL rather than by any of the returned JSON. For instance, if I make the request <code>http://www.service.com/GetObject?isSpecial=no</code> then I would like to specify that the attribute <code>isSpecial</code> is <code>NO</code>. Now I can probably accomplish something to this effect by adding multiple response descriptors, one for each of <code>GetObject?isSpecial=yes</code> and <code>GetObject?isSpecial=no</code>, but then I need to have a way of defining a special attribute mapping - I am not mapping attributes from key paths to key paths, but rather by request URL. How might I accomplish this?</p>