Fix issue #1388

[zzr93]

As issue #1388 reported, this situation is about implementing a deep copy method for a map, #1388 illustrates the problem cause by using Json Marshal/Unmarshal, here are some other choices:

1. Iterating over the old map and assign key-value pairs to the new map. But dealing with embed values can be ineffective and complex.
2. Using gob. The problem is custom structs need to be registered first, but this situation doesn't contain any custom struct.

It seems custom structs would not appear in this situation, so I chose gob finally.