Add tests for ID scalar input
Add tests for ID scalar input

This commit adds two tests cases for ID scalar input:
- a string literal
- an integer literal

Both of these literal types are covered by the GraphQL specification as
valid input for the ID scalar.

Reference the ID section of the spec for more information:
http://facebook.github.io/graphql/October2016/#sec-ID

chris-ramon/graphql-go,graph-gophers/graphql-go,neelance/graphql-go,neelance/graphql-go,graph-gophers/graphql-go,graph-gophers/graphql-go