title: Pandas adds a empty columns when saving
body: <p>I have a simply program in which i want to add a number to the end of a column</p>
  
  <pre><code>fajl = pd.read_csv('example.csv')
  print (fajl)
  fajl.at[len(fajl['Hrana']),'Hrana'] = 350
  print (fajl)
  fajl.to_csv('example.csv')
  </code></pre>
  
  <p>first print is :</p>
  
  <pre><code>   Hrana  Izlasci
  0   1641   6500.0
  1    910      NaN
  2   2100      NaN
  3    371      NaN
      Hrana  Izlasci
  0  1641.0   6500.0
  1   910.0      NaN
  2  2100.0      NaN
  3   371.0      NaN
  4   350.0      NaN
  </code></pre>
  
  <p>this is okey, this is what i expect it to do but when i start it the second time:</p>
  
  <pre><code>   Unnamed: 0   Hrana  Izlasci
  0           0  1641.0   6500.0
  1           1   910.0      NaN
  2           2  2100.0      NaN
  3           3   371.0      NaN
  4           4   350.0      NaN
     Unnamed: 0   Hrana  Izlasci
  0         0.0  1641.0   6500.0
  1         1.0   910.0      NaN
  2         2.0  2100.0      NaN
  3         3.0   371.0      NaN
  4         4.0   350.0      NaN
  5         NaN   350.0      NaN
  </code></pre>
  
  <p>It adds a new column and I dont know why. I am new to pandas so can someone explain why this is happening and how can I fix it?</p>