Add support for int64 IDs

[cfilby]

Based on the GraphQL Spec, Input ID types should be coerced into strings if provided as an integer during input. In a local GraphQL API we started getting "int64 is not a string" errors when trying to pass an integer in as a resolver ID parameter while using the default graphql.ID type. We were able to recreate this behavior using the gqlgen starwars example project as well. This error does not happen while using graphql.IntID type for marshalling.

This PR adds an int64 case to the MarshalID method, which appears to resolve the issue in my quick smoke test.

Let me know if you want me to update the formatting for the test, since it looks like it diverges from the repo style.

Related Issues:

• ID! validation is returning an error of type string. #798

I have:

• [*] Added tests covering the bug / feature (see testing)
• [*] Updated any relevant documentation (see docs)

[coveralls]

Coverage increased (+0.06%) to 62.01% when pulling 13c3d92 on cfilby:fix-int64-marshalling into dbc8842 on 99designs:master.

[frederikhors]

@cfilby is this working using go run github.com/99designs/gqlgen -v today?

I still have this message using int64: ID is incompatible with int64.