Handle the case if json tag was not overwritten

[borosr]

If the json tag was not present in the go_struct_tags option, the lib would use the default json tag, generated by protobuf.
#22

[PumpkinSeed]

Actually what you should do, you have to get the o.goStructTags, and if there is a tag from the override than 1. override the value if it exists in the o.goStructTags, 2. add it if it's not exists. The current solution somehow overcomplicate the problme. Correct me if I'm wrong and you are doing this.

[borosr]

Basically, my implementation is doing the same in the getAdditionalTags function, where the logic is:

1. Reading the goStructTags attached to the field - if empty, return with the default json tag
2. Parsing the passed goStructTags, but skipping those which are generated for gRPC
   At this point, we have an array full of non-existing and/or the overwritten json tag in it
3. Checking if the parsed array is containing json tag, if not, then add the default json tag to the parsed array