title: Numpy masked_array sum
body: <p>I would expect the result of a summation for a fully masked array to be zero, but instead "masked" is returned. How can I get the function to return zero?</p>
  
  <pre><code>&gt;&gt;&gt; a = np.asarray([1, 2, 3, 4])
  &gt;&gt;&gt; b = np.ma.masked_array(a, mask=~(a &gt; 2))
  &gt;&gt;&gt; b
  masked_array(data = [-- -- 3 4],
               mask = [ True  True False False],
         fill_value = 999999)
  
  &gt;&gt;&gt; b.sum()
  7
  &gt;&gt;&gt; b = np.ma.masked_array(a, mask=~(a &gt; 5))
  &gt;&gt;&gt; b
  masked_array(data = [-- -- -- --],
           mask = [ True  True  True  True],
     fill_value = 999999)
  
  
  &gt;&gt;&gt; b.sum()
  masked
  &gt;&gt;&gt; np.ma.sum(b)
  masked
  &gt;&gt;&gt; 
  </code></pre>
  
  <p>Here's another unexpected thing:</p>
  
  <pre><code>&gt;&gt;&gt; b.sum() + 3
  masked
  </code></pre>