Use leaf? check instead of !fields?

Author: rmosolgo

Diff for: lib/graphql/static_validation/rules/fields_will_merge.rb

@@ -269,7 +269,7 @@ def return_types_conflict?(type1, type2)
           end
         elsif type2.non_null?
           true
-        elsif !type1.kind.fields? && !type2.kind.fields?
+        elsif type1.kind.leaf? && type2.kind.leaf?
           type1 != type2
         else
           # One or more of these are composite types,