pg, default_sequence_name needs to return a string.

This is a reacon to d6c1205584 (commitcomment-7502487)
This backwards incompatibility was introduced with d6c12055 to fix #7516.
However both `connection.default_sequence_name` and `model.sequence_name` are public API.
The PostgreSQL adapter should honor the interface and return strings.

/cc @matthewd @chancancode

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   `default_sequence_name` from the PostgreSQL adapter returns a `String`.
+
+    *Yves Senn*
+
 *   Fixed a regression where whitespaces were stripped from DISTINCT queries in
     PostgreSQL.
 

activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb

@@ -281,9 +281,9 @@ module ActiveRecord
         def default_sequence_name(table_name, pk = nil) #:nodoc:
           result = serial_sequence(table_name, pk || 'id')
           return nil unless result
-          Utils.extract_schema_qualified_name(result)
+          Utils.extract_schema_qualified_name(result).to_s
         rescue ActiveRecord::StatementInvalid
-          PostgreSQL::Name.new(nil, "#{table_name}_#{pk || 'id'}_seq")
+          PostgreSQL::Name.new(nil, "#{table_name}_#{pk || 'id'}_seq").to_s
         end
 
         def serial_sequence(table, column)

activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb

@@ -134,18 +134,18 @@ module ActiveRecord
       end
 
       def test_default_sequence_name
-        assert_equal PostgreSQL::Name.new('public', 'accounts_id_seq'),
+        assert_equal 'public.accounts_id_seq',
           @connection.default_sequence_name('accounts', 'id')
 
-        assert_equal PostgreSQL::Name.new('public', 'accounts_id_seq'),
+        assert_equal 'public.accounts_id_seq',
           @connection.default_sequence_name('accounts')
       end
 
       def test_default_sequence_name_bad_table
-        assert_equal PostgreSQL::Name.new(nil, 'zomg_id_seq'),
+        assert_equal 'zomg_id_seq',
           @connection.default_sequence_name('zomg', 'id')
 
-        assert_equal PostgreSQL::Name.new(nil, 'zomg_id_seq'),
+        assert_equal 'zomg_id_seq',
           @connection.default_sequence_name('zomg')
       end
 
@@ -153,7 +153,7 @@ module ActiveRecord
         with_example_table do
           pk, seq = @connection.pk_and_sequence_for('ex')
           assert_equal 'id', pk
-          assert_equal @connection.default_sequence_name('ex', 'id'), seq
+          assert_equal @connection.default_sequence_name('ex', 'id'), seq.to_s
         end
       end
 
@@ -161,7 +161,7 @@ module ActiveRecord
         with_example_table 'code serial primary key' do
           pk, seq = @connection.pk_and_sequence_for('ex')
           assert_equal 'code', pk
-          assert_equal @connection.default_sequence_name('ex', 'code'), seq
+          assert_equal @connection.default_sequence_name('ex', 'code'), seq.to_s
         end
       end