title: Resample in pandas without changing date
body: <p>Suppose I download daily stock price data using pandas:</p>
  
  <pre><code>df = web.DataReader('YHOO', 'yahoo', '1/1/2004', '1/31/2004')['Close']
  </code></pre>
  
  <p>I want closing prices on the first trading day of each week. This is typically Monday, but will be Tuesday if Monday is a holiday.</p>
  
  <p>So I try resampling:</p>
  
  <pre><code>df.resample('W-MON', how='last')
  </code></pre>
  
  <p>and get this:</p>
  
  <pre><code>Date
  2004-01-05    46.900002
  2004-01-12    49.740002
  2004-01-19    48.110001
  2004-01-26    48.160000
  2004-02-02    46.980000
  </code></pre>
  
  <p>Pandas is very cleverly giving me data with only Mondays. But <code>2004-01-19</code> was a holiday, and is actually not in <code>df</code>:</p>
  
  <pre><code>Date
  2004-01-02    45.400002
  2004-01-05    46.900002
  ...
  2004-01-15    48.090000
  2004-01-16    48.110001
  2004-01-20    47.660000
  2004-01-21    47.380001
  ...
  </code></pre>
  
  <p>Is there a way to have it give the first date each week <em>from the dataframe</em> that is on or after a Monday? (So, the result above, except with <code>2004-01-19</code> replaced with <code>2004-01-20</code>.)</p>