Save a query when starting an isolated txn in Postgres

It's not a common operation, but still, no point running two queries
when one will do.

Author: matthewd

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

@@ -109,8 +109,7 @@ def begin_db_transaction # :nodoc:
         end
 
         def begin_isolated_db_transaction(isolation) # :nodoc:
-          begin_db_transaction
-          execute "SET TRANSACTION ISOLATION LEVEL #{transaction_isolation_levels.fetch(isolation)}"
+          execute("BEGIN ISOLATION LEVEL #{transaction_isolation_levels.fetch(isolation)}", "TRANSACTION")
         end
 
         # Commits a transaction.