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`.

1 files changed, 2 insertions, 10 deletions

diff --git a/activerecord/test/cases/relation_test.rb b/activerecord/test/cases/relation_test.rb
index d2859cbafd..82b071fb98 100644
--- a/activerecord/test/cases/relation_test.rb
+++ b/activerecord/test/cases/relation_test.rb
@@ -89,21 +89,13 @@ module ActiveRecord
 
     def test_create_with_value_with_wheres
       relation = Relation.new(Post, Post.arel_table, Post.predicate_builder)
-      relation.where!(id: 10)
-      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)
+      assert_equal({ "id" => 10 }, relation.scope_for_create)
 
       relation.create_with_value = { hello: "world" }
-      assert_equal({}, relation.scope_for_create)
+      assert_equal({ "hello" => "world", "id" => 10 }, relation.scope_for_create)
     end
 
     def test_bad_constants_raise_errors