Fix keras-team#821

`decay` in `SGD` is not a shared value, so it does not have a `get_value` method.

keras/optimizers.py

@@ -82,7 +82,7 @@ def get_config(self):
         return {"name": self.__class__.__name__,
                 "lr": float(self.lr.get_value()),
                 "momentum": float(self.momentum.get_value()),
-                "decay": float(self.decay.get_value()),
+                "decay": float(self.decay),
                 "nesterov": self.nesterov}