Fix connection leak when no connection open in the current thread (#22)

Follow-up for #21
Fixes #20 (specifically #20 (comment))

Method `connected?` checks if there are any connections open, even in a different thread,
while `active_connection?` checks for connection owned by current thread.

See:
 - https://api.rubyonrails.org/v5.2.3/classes/ActiveRecord/ConnectionAdapters/ConnectionPool.html#method-i-active_connection-3F
 - https://api.rubyonrails.org/v5.2.3/classes/ActiveRecord/ConnectionAdapters/ConnectionPool.html#method-i-connected-3F

lib/after_commit_everywhere.rb

@@ -125,7 +125,7 @@ def register_callback(connection: nil, name:, without_tx:, callback:)
     # Helper method to determine whether we're currently in transaction or not
     def in_transaction?(connection = nil)
       # Don't establish new connection if not connected: we apparently not in transaction
-      return false unless connection || ActiveRecord::Base.connection_pool.connected?
+      return false unless connection || ActiveRecord::Base.connection_pool.active_connection?
 
       connection ||= default_connection
       # service transactions (tests and database_cleaner) are not joinable

spec/after_commit_everywhere_spec.rb

@@ -23,7 +23,7 @@
     subject do
       example_class.new.after_commit do
         handler.call
-        expect(ActiveRecord::Base.connection.transaction_open?).to(be_falsey) if ActiveRecord::Base.connection_pool.connected?
+        expect(ActiveRecord::Base.connection.transaction_open?).to(be_falsey) if ActiveRecord::Base.connection_pool.active_connection?
       end
     end
 
@@ -186,6 +186,14 @@
         expect { subject }.not_to change { ActiveRecord::Base.connection_pool.connections.size }
       end
     end
+
+    context "when connection to the database has been established in another thread" do
+      before { ActiveRecord::Base.connection }
+
+      it "doesn't leak connections" do
+        expect { Thread.new { subject }.join }.not_to change { ActiveRecord::Base.connection_pool.connections.size }
+      end
+    end
   end
 
   describe "#before_commit" do