code: public Connection beginTransaction(ConnectionSource connectionSource) {
          return this.beginTransaction(connectionSource, java.sql.Connection.TRANSACTION_READ_COMMITTED);
      }
docs: Begins a transaction with isolation level {@link java.sql.Connection#TRANSACTION_READ_COMMITTED}. Every statement executed on the return {@link Connection}
  instance, will be executed in the transaction. It is very important to always call either the {@link org.sql2o.Connection#commit()}
  method or the {@link org.sql2o.Connection#rollback()} method to close the transaction. Use proper try-catch logic.
  @param connectionSource the {@link ConnectionSource} implementation substitution,
  that will be used instead of one from {@link Sql2o} instance.
  @return the {@link Connection} instance to use to run statements in the transaction.