`definition.go`: Increase test coverage up to 90%.

[chris-ramon]

Current

• Currently the test coverage of github.com/graphql-go/graphql/definition.go is at (84.2%).

Expected

• We want to increase it up to 90%.

[chris-ramon]

Test Coverage

go clean -testcache && go test -cover -coverprofile=coverage.out ./... && go tool cover -html=coverage.out -o coverage.html; sed -i'' -e's/black/whitesmoke/g' coverage.html && open coverage.html