Bug: FieldFunc selectionSet argument is always nil

[Dorokhov]

FieldFunc() should receive SelectionSet for particular node so that the resolver could build the appropriate query based on user's selection, but selectionSet is always nil.

Code example:

query.FieldFunc("viewer", func(ctx context.Context, selectionSet *graphql.SelectionSet) *User {
	// selectionSet is always nil
})