title: Pandas read_excel sometimes creates index even when index_col=None
body: <p>I'm trying to read an excel file into a data frame and I want set the index later, so I don't want pandas to use column 0 for the index values.</p>
  <p>By default (<code>index_col=None</code>), it shouldn't use column 0 for the index but I find that if there is <strong>no</strong> value in cell A1 of the worksheet it will.</p>
  <p>Is there any way to over-ride this behaviour (I am loading many sheets that have no value in cell A1)?</p>
  <p>This works as expected when test1.xlsx has the value &quot;DATE&quot; in cell A1:</p>
  <pre><code>In [19]: pd.read_excel('test1.xlsx')                                             
  Out[19]: 
                   DATE         A         B         C
  0 2018-01-01 00:00:00  0.766895  1.142639  0.810603
  1 2018-01-01 01:00:00  0.605812  0.890286  0.810603
  2 2018-01-01 02:00:00  0.623123  1.053022  0.810603
  3 2018-01-01 03:00:00  0.740577  1.505082  0.810603
  4 2018-01-01 04:00:00  0.335573 -0.024649  0.810603
  </code></pre>
  <p>But when the worksheet has no value in cell A1, it automatically assigns column 0 values to the index:</p>
  <pre><code>In [20]: pd.read_excel('test2.xlsx', index_col=None)                             
  Out[20]: 
                              A         B         C
  2018-01-01 00:00:00  0.766895  1.142639  0.810603
  2018-01-01 01:00:00  0.605812  0.890286  0.810603
  2018-01-01 02:00:00  0.623123  1.053022  0.810603
  2018-01-01 03:00:00  0.740577  1.505082  0.810603
  2018-01-01 04:00:00  0.335573 -0.024649  0.810603
  </code></pre>
  <p>This is not what I want.</p>
  <p>Desired result: Same as first example (but with 'Unnamed' as the column label perhaps).</p>
  <p><a href="https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.read_excel.html" rel="noreferrer">Documentation</a> says</p>
  <blockquote>
  <p>index_col : int, list of int, default None.</p>
  <p>Column (0-indexed) to use as the row labels of the DataFrame. Pass None if there is no such column.</p>
  </blockquote>