go_struct_tags removes json tag

[PumpkinSeed]

If we use go_struct_tags to define the additional tags it is remove the tag if it's not on the list. Special case when we add it like this: (go_struct_tags) = "form=first_name", because this will remove the json tag created by the proto generator.

Make sure that the go_struct_tags only add or overwrite tags but never remove them.