python3 bugfix

Author: Martin Journois

folium/plugins/geo_json.py

@@ -30,7 +30,7 @@ def __init__(self, data):
         """
         super(GeoJson, self).__init__()
         self.plugin_name = 'GeoJson'
-        if type(data) is file:
+        if 'read' in dir(data):
             self.data = data.read()
         elif type(data) is dict:
             self.data = json.dumps(data)