Make SEQUEL_DEFAULT_CONNECTION_POOL environment variable handle sharding

If the :servers option is present, but the environment variable
is not for a sharded pool, automatically switch to a sharded
version.  This allows all specs to pass with
SEQUEL_DEFAULT_CONNECTION_POOL=timed_queue on Ruby 3.2.

lib/sequel/connection_pool.rb

@@ -65,7 +65,8 @@ def connection_pool_class(opts)
 
         pc
       elsif pc = ENV['SEQUEL_DEFAULT_CONNECTION_POOL']
-        connection_pool_class(:pool_class=>ENV['SEQUEL_DEFAULT_CONNECTION_POOL'])
+        pc = "sharded_#{pc}" if opts[:servers] && !pc.start_with?('sharded_')
+        connection_pool_class(:pool_class=>pc)
       else
         pc = if opts[:single_threaded]
           opts[:servers] ? :sharded_single : :single

spec/core/connection_pool_spec.rb

@@ -82,6 +82,10 @@ def available_connections(server=:default)
       ENV['SEQUEL_DEFAULT_CONNECTION_POOL'] = 'sharded_threaded'
       pool = Sequel::ConnectionPool.get_pool(mock_db.call)
       pool.must_be_instance_of Sequel::ShardedThreadedConnectionPool
+
+      ENV['SEQUEL_DEFAULT_CONNECTION_POOL'] = 'single'
+      pool = Sequel::ConnectionPool.get_pool(mock_db.call, :servers=>{})
+      pool.must_be_instance_of Sequel::ShardedSingleConnectionPool
     ensure
       ENV.delete('SEQUEL_DEFAULT_CONNECTION_POOL')
     end