title: storing [NSNull null] values in NSUserDefaults, from JSON serialization, causes unwanted exceptions
body: <p>I've got an app where I use a JSON based API.  As part of JSON, often values are set to "null".  This may be common:</p>
  
  <pre><code>{"data":["one","two","three"],"name":null,otherstuff:10}
  </code></pre>
  
  <p>Recently I've tried to store a misc NSDictionary hierarchy, converted from a JSON object, in NSUserDefaults.  Unfortunately it causes an exception if there is null data, converted in IOS to [NSNull null].  Apparently that can't be saved in prefs. </p>
  
  <p>I was wondering if anyone has worked around this before?  I tried to add some logic to remove all null values from the JSON first, with limited success, but it seems inappropriate to have to modify the data before storing it.  Is there a better way to handle this?</p>