title: Keras Tokenizer num_words doesn't seem to work
body: <pre><code>&gt;&gt;&gt; t = Tokenizer(num_words=3)
  &gt;&gt;&gt; l = ["Hello, World! This is so&amp;#$ fantastic!", "There is no other world like this one"]
  &gt;&gt;&gt; t.fit_on_texts(l)
  &gt;&gt;&gt; t.word_index
  {'fantastic': 6, 'like': 10, 'no': 8, 'this': 2, 'is': 3, 'there': 7, 'one': 11, 'other': 9, 'so': 5, 'world': 1, 'hello': 4}
  </code></pre>
  
  <p>I'd have expected <code>t.word_index</code> to have just the top 3 words. What am I doing wrong?</p>