Don't cache scope_for_create

I investigated where `scope_for_create` is reused in tests with the
following code:

```diff
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -590,6 +590,10 @@ def where_values_hash(relation_table_name = table_name)
     end

     def scope_for_create
+      if defined?(@scope_for_create) && @scope_for_create
+        puts caller
+        puts "defined"
+      end
       @scope_for_create ||= where_values_hash.merge!(create_with_value.stringify_keys)
     end
```

It was hit only `test_scope_for_create_is_cached`. This means that
`scope_for_create` will not be reused in normal use cases. So we can
remove caching `scope_for_create` to respect changing `where_clause` and
`create_with_value`.

activerecord/lib/active_record/associations/association.rb

@@ -171,7 +171,7 @@ module ActiveRecord
         skip_assign = [reflection.foreign_key, reflection.type].compact
         assigned_keys = record.changed_attribute_names_to_save
         assigned_keys += except_from_scope_attributes.keys.map(&:to_s)
-        attributes = scope_for_create.except(*(assigned_keys - skip_assign))
+        attributes = scope_for_create.except!(*(assigned_keys - skip_assign))
         record.send(:_assign_attributes, attributes) if attributes.any?
         set_inverse_instance(record)
       end

activerecord/lib/active_record/associations/singular_association.rb

@@ -31,7 +31,7 @@ module ActiveRecord
 
       private
         def scope_for_create
-          super.except(klass.primary_key)
+          super.except!(klass.primary_key)
         end
 
         def find_target

activerecord/lib/active_record/relation.rb

@@ -556,7 +556,7 @@ module ActiveRecord
     end
 
     def reset
-      @to_sql = @scope_for_create = @arel = @loaded = @should_eager_load = nil
+      @to_sql = @arel = @loaded = @should_eager_load = nil
       @records = [].freeze
       @offsets = {}
       self
@@ -590,7 +590,7 @@ module ActiveRecord
     end
 
     def scope_for_create
-      @scope_for_create ||= where_values_hash.merge!(create_with_value.stringify_keys)
+      where_values_hash.merge!(create_with_value.stringify_keys)
     end
 
     # Returns true if relation needs eager loading.

activerecord/test/cases/relation_test.rb

@@ -89,23 +89,15 @@ module ActiveRecord
 
     def test_create_with_value_with_wheres
       relation = Relation.new(Post, Post.arel_table, Post.predicate_builder)
+      assert_equal({}, relation.scope_for_create)
+
       relation.where!(id: 10)
+      assert_equal({ "id" => 10 }, relation.scope_for_create)
+
       relation.create_with_value = { hello: "world" }
       assert_equal({ "hello" => "world", "id" => 10 }, relation.scope_for_create)
     end
 
-    # FIXME: is this really wanted or expected behavior?
-    def test_scope_for_create_is_cached
-      relation = Relation.new(Post, Post.arel_table, Post.predicate_builder)
-      assert_equal({}, relation.scope_for_create)
-
-      relation.where!(id: 10)
-      assert_equal({}, relation.scope_for_create)
-
-      relation.create_with_value = { hello: "world" }
-      assert_equal({}, relation.scope_for_create)
-    end
-
     def test_bad_constants_raise_errors
       assert_raises(NameError) do
         ActiveRecord::Relation::HelloWorld