title: Replace values in masked numpy array not working
body: <p>I have the foll. masked array in numpy called arr with shape (50, 360, 720):</p>
  
  <pre><code>masked_array(data =
   [[-- -- -- ..., -- -- --]
   [-- -- -- ..., -- -- --]
   [-- -- -- ..., -- -- --]
   ..., 
   [-- -- -- ..., -- -- --]
   [-- -- -- ..., -- -- --]
   [-- -- -- ..., -- -- --]],
               mask =
   [[ True  True  True ...,  True  True  True]
   [ True  True  True ...,  True  True  True]
   [ True  True  True ...,  True  True  True]
   ..., 
   [ True  True  True ...,  True  True  True]
   [ True  True  True ...,  True  True  True]
   [ True  True  True ...,  True  True  True]],
         fill_value = 1e+20)
  </code></pre>
  
  <p>It has the foll. data in arr[0]:</p>
  
  <pre><code>arr[0].data
  
  array([[-999., -999., -999., ..., -999., -999., -999.],
         [-999., -999., -999., ..., -999., -999., -999.],
         [-999., -999., -999., ..., -999., -999., -999.],
         ..., 
         [-999., -999., -999., ..., -999., -999., -999.],
         [-999., -999., -999., ..., -999., -999., -999.],
         [-999., -999., -999., ..., -999., -999., -999.]])
  </code></pre>
  
  <p>-999. is the missing_value and I want to replace it by 0.0. I do this:</p>
  
  <pre><code>arr[arr == -999.] = 0.0
  </code></pre>
  
  <p>However, arr remains the same even after this operation. How to fix this?</p>