[Schema Inaccuracy] git-user's date type should be date-time

[jessfraz]

Schema Inaccuracy

The type git-user's date type to be date-time that way clients that are generated can parse it correctly. Ex generated client: https://docs.rs/octorust/0.1.5/octorust/types/struct.GitUser.html

I mean its fine now that it is a string its just not totally correct and if you all denoted it as a datetime then I could automatically parse it as such.