title: Pandas replace function does not work on Series of strings
body: <p>pandas.DataFrame.replace doesn't replace string.</p>
  <pre><code>dic = {'Text': ['i8am going to school', 'i8am a very good boy']}
  df = pd.DataFrame(dic)
  d = df['Text'].replace(to_replace='i8am', value='i am')
  print(d)
  </code></pre>
  <p>Expected output:</p>
  <pre><code>I am going to school
  I am a very good boy
  </code></pre>