Change the internal transaction state to be closed when the transaction is finalized

Before we were using the null pattern to calculate if the transaction
is open or not, but there are cases like when the transaction is invalidated
that we keep the transaction object around, and just change the state.

The previous behavior made us miss cases where the transaction aren't
not really open in our code. See 2f571a5.

The public facing transaction object already had the behavior we are
implementing here, so we are just aligning the internal transaction
object to have the same behavior.

Author: rafaelfranca

activerecord/lib/active_record.rb

@@ -550,7 +550,7 @@ def self.all_open_transactions # :nodoc:
       if active_connection = pool.active_connection
         current_transaction = active_connection.current_transaction
 
-        if current_transaction.open? && current_transaction.joinable? && !current_transaction.state.invalidated?
+        if current_transaction.open? && current_transaction.joinable?
           open_transactions << current_transaction
         end
       end

activerecord/lib/active_record/connection_adapters/abstract/transaction.rb

@@ -175,11 +175,11 @@ def dirty?
       end
 
       def open?
-        true
+        !closed?
       end
 
       def closed?
-        false
+        @state.finalized?
       end
 
       def add_record(record, ensure_finalize = true)

activerecord/lib/active_record/transaction.rb

@@ -112,7 +112,7 @@ def open?
 
     # Returns true if the transaction doesn't exist or is finalized.
     def closed?
-      @internal_transaction.nil? || @internal_transaction.state.finalized?
+      @internal_transaction.nil? || @internal_transaction.closed?
     end
 
     alias_method :blank?, :closed?