The way we generate map types today is map[string]T, where T is a
non-pointer type. This means the value receiver version of
MarshalJSON does not fullfill the Marshaler interface. This is turn
means that MarshalJSON is not called in these cases so ForceSendFields
overrides do not work. To fix this we can switch the impl to a value
reciever which fullfills both value and pointer types. This is not a
breaking change for the API, but there is a slight behaviour change
if someone was calling MarshalJSON directly. Nil *T's that call
MarshalJSON directly will now panic instead of returning JSON `null`.
This seems like unlikely usage and worth breaking to fix a bug in
common usage.

…o json-marshall