title: json.dump() gives me "TypeError: keys must be a string"
body: <p>I have some very simple code that takes a dictionary with a tuple as a key and turns it into json:</p>
  
  <pre><code>In [11]:
  
  import simplejson as json
  In [12]:
  
  data = {('category1', 'category2'): 4}
  In [13]:
  
  json.dumps(data)
  </code></pre>
  
  <p>However, running the code gives me:</p>
  
  <pre><code>TypeError: keys must be a string
  </code></pre>
  
  <p>I have tried str()'ing the keys and everything else I can find, but without luck.</p>