title: Resample with DateOffset can give NotImplementedError
body: <p>Docs for <a href="https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.resample.html" rel="noreferrer"><code>resample</code></a> say it can take a <a href="https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.tseries.offsets.DateOffset.html" rel="noreferrer"><code>DateOffset</code></a> as the <strong>rule</strong>.<br/>
  But trying this out sometimes results in a <code>NotImplementedError</code>.</p>
  
  <ul>
  <li>Passing a string e.g. <code>resample('2M')</code> works.</li>
  <li>The equivalent <code>resample(pd.offsets.MonthEnd(2))</code> also works.</li>
  <li>But the similar (though different) <code>resample(pd.offsets.DateOffset(months=2))</code> fails.</li>
  </ul>
  
  <p>Here's code to reproduce:</p>
  
  <pre class="lang-py prettyprint-override"><code>idx = pd.date_range('20190101', periods=14, freq='3W')
  s   = pd.Series(range(len(idx)), index=idx)
  s.resample(pd.offsets.DateOffset(months=2)).mean()
  </code></pre>
  
  <p>Trace:</p>
  
  <pre><code>Traceback (most recent call last):
      ....
    File "....core\generic.py", line 8449, in resample
      level=level,
    File "....core\resample.py", line 1305, in resample
      tg = TimeGrouper(**kwds)
    File "....core\resample.py", line 1378, in __init__
      rule = freq.rule_code
    File "....tseries\offsets.py", line 442, in rule_code
      return self._prefix
    File "....tseries\offsets.py", line 438, in _prefix
      raise NotImplementedError("Prefix not defined")
  NotImplementedError: Prefix not defined
  </code></pre>
  
  <p>Am I right that <a href="https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.resample.html" rel="noreferrer"><code>resample</code></a> supports only certain <a href="https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.tseries.offsets.DateOffset.html" rel="noreferrer"><code>DateOffsets</code></a>?<br/>
  <br/>
  What kinds doesn't/does it support?</p>