viz: hotfix for saving in cache (apache#1922)

Fix cache set after c14c7ed update json serializer behaviour
but forget to update one call site.

While at it move payload serialization outside of catch all try
block since we want to see explosions if any.

Fix apache#1910

superset/viz.py

@@ -379,10 +379,13 @@ def get_payload(self, force=False):
             payload['cached_dttm'] = datetime.now().isoformat().split('.')[0]
             logging.info("Caching for the next {} seconds".format(
                 cache_timeout))
+            data = json.dumps(
+                payload,
+                default=utils.json_int_dttm_ser, ignore_nan=True
+            )
+            if PY3:
+                data = bytes(data, 'utf-8')
             try:
-                data = self.json_dumps(payload)
-                if PY3:
-                    data = bytes(data, 'utf-8')
                 cache.set(
                     cache_key,
                     zlib.compress(data),