title: imdbpy: do not get full writer list from get_movie['writer']
body: <p>Checking for the series Firefly <a href="http://www.imdb.com/title/tt0303461/" rel="nofollow">http://www.imdb.com/title/tt0303461/</a>.
  Given the code:</p>
  
  <pre><code>from imdb import IMDb
  parser = IMDb()
  parsed = parser.get_movie('0303461')
  list_writers = parsed['writer']
  </code></pre>
  
  <p>The list parsed['writer'] appears to ignore any writers with less than two episode to their name. 
  Calling:</p>
  
  <pre><code>parser.update(parsed,'writer')
  </code></pre>
  
  <p>Results in the error:</p>
  
  <blockquote>
    <p>2015-10-06 12:24:05,877 ERROR [imdbpy] /usr/local/lib/python2.7/dist-packages/imdb/<strong>init</strong>.py:768: unknown information set "writer"</p>
  </blockquote>
  
  <p>Is this correct functionality, and the full list can only be obtained through the sql database? Or am I doing something wrong?</p>
  
  <p>Responding to the @davide-alberani's commend comment below:</p>
  
  <p>I agree that the fetch was unnecessary, however with or without it, it does not get the entire record that can be seen from imdb itself. Running parsed.get('writer'), I get the following name list: 
      ["Whedon, Joss", "Minear, Tim", "Edlund, Ben", "Molina, Jose"] </p>
  
  <p>The series writing credits <a href="http://www.imdb.com/title/tt0303461/fullcredits?ref_=tt_ql_1" rel="nofollow">here</a>, show the following additional writers: </p>
  
  <ul>
  <li>Cheryl Cain   ... (1 episode, 2002) </li>
  <li>Jane Espenson ... (1 episode, 2002) </li>
  <li>Drew Z. Greenberg ... (1 episode, 2002) </li>
  <li>Brett Matthews    ... (1 episode, 2003) </li>
  </ul>