fix tests to handle default projection to mercator

Author: andrewgiessel

tests/folium_tests.py

@@ -484,21 +484,26 @@ def test_fit_bounds(self):
     def test_image_overlay(self):
         """Test image overlay"""
         from numpy.random import random
-        from folium.utilities import write_png
+        from folium.utilities import write_png, geodetic_to_mercator
         import base64
 
         data = random((100,100))
         png_str = write_png(data)
         with open('data.png', 'wb') as f:
             f.write(png_str)
-        inline_image_url = "data:image/png;base64,"+base64.b64encode(png_str).decode('utf-8')
+
+        image_url = 'data.png'
+        inline_image_url = ("data:image/png;base64," +
+                            base64.b64encode(write_png(geodetic_to_mercator(data))).decode('utf-8'))
 
         image_tpl = self.env.get_template('image_layer.js')
         image_name = 'Image_Overlay'
         image_opacity = 0.25
-        image_url = 'data.png'
+
         min_lon, max_lon, min_lat, max_lat = -90.0, 90.0, -180.0, 180.0
         image_bounds = [[min_lon, min_lat], [max_lon, max_lat]]  
+
+        # Test the external png.
 
         image_rendered = image_tpl.render({'image_name': image_name,
                                            'image_url': image_url,
@@ -509,6 +514,7 @@ def test_image_overlay(self):
         self.map.image_overlay(data, filename=image_url)
         assert image_rendered in self.map.template_vars['image_layers']
 
+        # Test the inline png.
 
         image_rendered = image_tpl.render({'image_name': image_name,
                                            'image_url': inline_image_url,