title: Index not the same between writing and reading excel file
body: <p>Here is the problem. I have a data frame that looks something like this </p>
  
  <pre><code>df1 = pd.read_excel('excelA.xlsx')
  df2 = pd.read_excel('excelB.xlsx')
  
  ind = df1['some_col']
  cols = df2.columns
  
  # Using index from df1 and columns from df2 create a new df
  res_df = pd.DataFrame(index=ind, columns=cols)
  
  # Inserting values from df2 into new df
  for c in cols:
      for j in range(len(df2[c])):
          res_df[c].iloc[j] = df2[c].iloc[j]
  
  print(res_df.index) // Index([something, something2, something3 ... ])
  
  writer = pd.ExcelWriter('test.xlsx', engine='xlsxwriter')
  res_df.to_excel(writer, sheet_name='Sheet1', index=True) 
  writer.close()
  </code></pre>
  
  <p>Now when I try to read the file</p>
  
  <pre><code>df = pd.read_excel('test.xlsx', dtype=str)
  print(df.index) // RangeIndex(start=0, stop=1000, step=1)
  </code></pre>
  
  <p>Can anyone tell me what is happening here?</p>