Do not try to set the foreign_key again on has_many through

Integration tests are inside protected_attributes test suite.

Fixes #15496
Fixes rails/protected_attributes#35

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -89,7 +89,7 @@ def build_through_record(record)
         end
 
         def through_scope_attributes
-          scope.where_values_hash(through_association.reflection.name.to_s)
+          scope.where_values_hash(through_association.reflection.name.to_s).except!(through_association.reflection.foreign_key)
         end
 
         def save_through_record(record)