Extension on mint message

[Morgandri1]

Why is Extension on MintMsg? Shouldnt the extensions be global throughout the contract?

If not, Extension should be Option<T> instead of T as this will make development easier.

[taitruong]

Imo similar to Response<T = Empty>. What we can do is defining ExecuteMsg<T=Empty, E=Empty>. Would simplify things.

[Morgandri1]

Yeah, and I found that you can define extension as Option<Empty>, but that’s more steps that are subjectively unnecessary, especially when trying to write clean and concise tests 

[taitruong]

Fixed in #161