title: value error in python statsmodels.tsa.seasonal
body: <p>I have this dataframe with date time indices:</p>
  <pre><code>ts_log:
  </code></pre>
  <pre><code>date    price_per_unit
  2013-04-04  12.762369
  2013-04-05  12.777120
  2013-04-06  12.773146
  2013-04-07  12.780774
  2013-04-08  12.786835
  </code></pre>
  <p>I have this piece of code for <code>decomposition</code></p>
  <pre><code>from statsmodels.tsa.seasonal import seasonal_decompose
  decomposition = seasonal_decompose(ts_log)
  
  trend = decomposition.trend
  seasonal = decomposition.seasonal
  residual = decomposition.resid
  </code></pre>
  <p>but in the line <code>    decomposition = seasonal_decompose(ts_log)</code>
  i got this error :</p>
  <pre><code>ValueError: You must specify a freq or x must be a pandas object with a timeseries index
  </code></pre>
  <p>Where is the problem?</p>