title: Numpy.ma polyfit function for masked arrays crashes on integer input
body: <p>The numpy polynomial fit function for masked arrays, <code>ma.polyfit</code>, crashes on integer iput:</p>
  
  <pre><code>import numpy.ma as ma
  x = ma.arange(2)
  y = ma.arange(2)
  p1 = ma.polyfit(np.float32(x), y, deg=1)
  p2 = ma.polyfit(           x , y, deg=1)
  </code></pre>
  
  <p>The last line results in an error:</p>
  
  <pre><code>ValueError: data type &lt;type 'numpy.int64'&gt; not inexact
  </code></pre>
  
  <p>Why can't I fit data with integer x-values (it's no problem with the normal <code>numpy.polyfit</code> function), is this a (known) bug?</p>