title: cannot change datatype from "from_dict" of dataframe
body: <p>After <code>df['a'].astype(float)</code> from <code>from_dict</code> cannot be sum the column.<br />
  I expected the <code>sum(a)</code> should be 15 not 159 and I just want to show <code>15</code> only not whole dataframe table</p>
  <p><strong>code</strong></p>
  <pre><code>x = [{'a': '1','b1': '2','b2': '3', 'c': '4'},{'a': '5','b1': '6','b2': '7','c': '8'},{'a': '9','b1': '10','b2': '11','c': '12'}]
  import pandas as pd
  df = pd.DataFrame.from_dict(x)
  
  df['a'] = df['a'].astype(float)
  df['total'] = df['a'].sum()
  df
  </code></pre>
  <p><strong>code output:</strong></p>
  <pre><code>    a   b1  b2  c   total
  0   1.0 2   3   4   15.0
  1   5.0 6   7   8   15.0
  2   9.0 10  11  12  15.0
  </code></pre>
  <p><strong>Data type</strong></p>
  <pre><code>df['a'].astype(float)
  df.dtypes
  
  a        float64
  b1       object
  b2       object
  c        object
  total    object
  dtype: object
  </code></pre>
  <p><strong>Expected Result</strong></p>
  <pre><code>15
  </code></pre>