Update Python flask default value (#361)

* update python flask default value

* update python flask samples

samples/server/petstore/flaskConnexion-python2/openapi_server/encoder.py

@@ -0,0 +1,20 @@
+from connexion.apps.flask_app import FlaskJSONEncoder
+import six
+
+from openapi_server.models.base_model_ import Model
+
+
+class JSONEncoder(FlaskJSONEncoder):
+    include_nulls = False
+
+    def default(self, o):
+        if isinstance(o, Model):
+            dikt = {}
+            for attr, _ in six.iteritems(o.openapi_types):
+                value = getattr(o, attr)
+                if value is None and not self.include_nulls:
+                    continue
+                attr = o.attribute_map[attr]
+                dikt[attr] = value
+            return dikt
+        return FlaskJSONEncoder.default(self, o)