title: CountVectorizer: "I" not showing up in vectorized text
body: <p>I'm new to scikit-learn, and currently studying Naïve Bayes (Multinomial). Right now, I'm working on vectorizing text from sklearn.feature_extraction.text, and for some reason, when I vectorize some text, the word "I" doesn't show up in the outputted array.</p>
  
  <p>Code:</p>
  
  <pre><code>x_train = ['I am a Nigerian hacker', 'I like puppies']
  
  # convert x_train to vectorized text
  vectorizer_train = CountVectorizer(min_df=0)
  vectorizer_train.fit(x_train)
  x_train_array = vectorizer_train.transform(x_train).toarray()
  
  # print vectorized text, feature names
  print x_train_array
  print vectorizer_train.get_feature_names()
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>1 1 0 1 0
  0 0 1 0 1
  [u'am', u'hacker', u'like', u'nigerian', u'puppies']
  </code></pre>
  
  <p>Why doesn't "I" seem to show up in the feature names? When I change it to "Ia" or something else like that, it does show up.</p>