Fix incorrect removal of current_shard in establish_connection

If we enter a `connected_to` block and call `establish_connection` like
the test added here we need to ensure that `shard: current_shard` is passed
to the handler, otherwise the connection will be established on
`default` not on `shard_one`.

Co-authored-by: John Crepezzi <john.crepezzi@gmail.com>

activerecord/lib/active_record/connection_handling.rb

@@ -49,7 +49,7 @@ module ConnectionHandling
     def establish_connection(config_or_env = nil)
       config_or_env ||= DEFAULT_ENV.call.to_sym
       db_config, owner_name = resolve_config_for_connection(config_or_env)
-      connection_handler.establish_connection(db_config, owner_name: owner_name)
+      connection_handler.establish_connection(db_config, owner_name: owner_name, shard: current_shard)
     end
 
     # Connects a model to the databases specified. The +database+ keyword

activerecord/test/cases/connection_adapters/connection_handlers_sharding_db_test.rb

@@ -25,6 +25,16 @@ def teardown
       end
 
       unless in_memory_db?
+        def test_establishing_a_connection_in_connected_to_block_uses_current_role_and_shard
+          ActiveRecord::Base.connected_to(shard: :shard_one) do
+            db_config = ActiveRecord::Base.configurations.configs_for(env_name: "arunit", name: "primary")
+            ActiveRecord::Base.establish_connection(db_config)
+            assert_nothing_raised { Person.first }
+
+            assert_equal [:default, :shard_one], ActiveRecord::Base.connection_handlers[:writing].send(:owner_to_pool_manager).fetch("ActiveRecord::Base").instance_variable_get(:@name_to_pool_config).keys
+          end
+        end
+
         def test_establish_connection_using_3_levels_config
           previous_env, ENV["RAILS_ENV"] = ENV["RAILS_ENV"], "default_env"