title: Using numpy.rate, on numpy array returns nan's unexpectedly
body: <p>I'm using np.rate function to calculate simple growth rates. I'm trying to use vector operations. This works:</p>
  
  <pre><code>b = np.arange(1,10)
  np.rate(1,0,-b[:-1],b[1:])
  </code></pre>
  
  <p>This returns:<BR>
  array([ 1.        ,  0.5       ,  0.33333333,  0.25      ,  0.2       ,
          0.16666667,  0.14285714,  0.125     ])</p>
  
  <p>However, this doesn't work as I'm expecting:</p>
  
  <pre><code>b = np.arange(10)
  np.rate(1,0,-b[:-1],b[1:])
  </code></pre>
  
  <p>Returns:<BR>
  array([ nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan])</p>
  
  <p>Any thoughts to best remedy this? I'm assuming the 0 in the first index position is giving the np.rate function some problems but I'm not sure what the best way to correct this is. Your help is appreciated.</p>