title: cmap.set_bad() not showing any effect with pcolor()
body: <p>I'm trying to use pcolor on a masked array. I would like masked elements
  to show up in a special color. I have written some code, but it does not
  seem to work:</p>
  
  <pre><code>import matplotlib as mpl
  import matplotlib.pyplot as plt
  
  from numpy import linspace
  from numpy.random import randn
  from numpy.ma import masked_invalid
  
  D = randn(12*72).reshape((12,72))
  D[4,:] = nan
  D[6,6] = nan
  
  D = masked_invalid(D)
  
  cmap = mpl.cm.bwr
  c    map.set_bad('k', 1.)
  
  xbin = linspace(0, 12, 13)
  ybin = linspace(-90, 90, 73)
  
  fig = plt.figure()
  spl = fig.add_subplot(111)
  pl = spl.pcolor(xbin, ybin, D.T, cmap=cmap, edgecolors='none',
                  vmin=-5, vmax=5)
  </code></pre>