title: NSMutableArray Sorting Issue
body: <p>In my main file, I define NSMutableArray *creditList;</p>
  
  <p>In my loop (that increments myCount until finished) to populate the array with movie information from the JSON response, I have:</p>
  
  <pre><code>creditType = fetchedData[@"cast"][myCount][@"media_type"];
  creditTitle = fetchedData[@"cast"][myCount][@"title"];
  creditCharacter = fetchedData[@"cast"][myCount][@"character"];
  creditReleasedate = fetchedData[@"cast"][myCount][@"release_date"];
  creditReleasedate = fetchedData[@"cast"][myCount][@"release_date"];
  </code></pre>
  
  <p>The array populates as you would expect, but now it needs to be sorted by release date.</p>
  
  <p>I have tried:</p>
  
  <pre><code>NSSortDescriptor *ageDescriptor = [[NSSortDescriptor alloc] initWithKey:@"release_date" ascending:YES];
  NSArray *sortDescriptors = @[ageDescriptor];
  NSArray *sortedArrayByReleaseYear = [creditList sortedArrayUsingDescriptors:sortDescriptors];
  </code></pre>
  
  <p>However when the code runs, as soon as it gets to the sorting code I get:</p>
  
  <pre><code>*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[&lt;__NSCFString 0x7ca30db0&gt; valueForUndefinedKey:]: this class is not key value coding-compliant for the key release_date.'
  </code></pre>
  
  <p>I have tried other sorting methods from this site but I can't seem to get any of them to run without errors.  Extra kudos points to whomever can help thanks!</p>