Fixed bug for generating images with transparent background.

The contour used was lacking a transparency channel.

wordcloud/wordcloud.py

@@ -715,12 +715,16 @@ def _draw_contour(self, img):
         contour = Image.fromarray(contour)
         contour = contour.filter(ImageFilter.GaussianBlur(radius=radius))
         contour = np.array(contour) > 0
-        contour = np.dstack((contour, contour, contour))
+        if img.mode == 'RGBA':
+            contour = np.dstack((contour, contour, contour, contour))
+        else:
+            contour = np.dstack((contour, contour, contour))
 
         # color the contour
         ret = np.array(img) * np.invert(contour)
-        if self.contour_color != 'black':
-            color = Image.new(img.mode, img.size, self.contour_color)
+
+        if self.contour_color is not None:
+            color = np.array(Image.new(img.mode, img.size, self.contour_color))
             ret += np.array(color) * contour
 
         return Image.fromarray(ret)