title: apply.weekly() does not calculate the date correctly
body: <p>I'm trying to aggregate my data using the <code>apply.weekly()</code> function in xts. However, when I do so, one week has 8 days rather than 7 days. Here is the output of <code>apply.weekly()</code> function:</p>
  
  <pre><code>2013-06-01 0.0000000
  2013-06-08 0.5718182
  2013-06-15 0.3365385
  2013-06-22 0.7534615
  2013-06-30 0.7467568
  2013-07-07 0.7996000
  2013-07-13 0.2811111
  2013-07-21 0.3743750
  2013-07-28 0.2142857
  2013-08-04 0.5664516
  2013-08-11 0.5790000
  2013-08-18 0.3404255
  2013-08-24 0.5240625
  2013-08-31 1.1986207
  2013-09-06 4.8233333
  </code></pre>
  
  <p>As you can see, in above output the week between "2013-06-22" and "2013-06-30" has 8 days and it changes all the consecutive indexes in my program. Interestingly, the week between "2013-08-18" and "2013-08-24" has only 6 days.</p>
  
  <p>Any idea how can I fix this?</p>
  
  <p>EDIT: here is a glimpse of the data</p>
  
  <pre><code>index       price
  2013-06-01  0.000
  2013-06-06  5.290
  2013-06-08  1.000
  2013-06-13  6.750
  2013-06-15  1.000
  2013-06-15  1.000
  2013-06-17  1.000
  2013-06-19  1.000
  2013-06-20  1.000
  2013-06-21  16.590
  2013-06-24  5.990
  2013-06-25  4.650
  2013-06-26  1.000
  2013-06-26  11.990
  2013-06-27  1.000
  2013-06-29  1.000
  2013-06-29  1.000
  2013-06-30  1.000
  </code></pre>
  
  <p>After loading this data, I'm just trying to use the following command:</p>
  
  <pre><code>price_weekly &lt;- apply.weekly(dt$price,mean)
  </code></pre>