db_set_active not working with node_save

Idea is coming from,

We do use *db_set_active* to switch database, now suppose I have a multiple drupal site and it's required to execute node_save
in this conjunction as db_transaction have a call at the initial line of node_save without considering the different *key* so it's always fallback to default only.

So instead of using *db_set_active* enforce to switch using an optional variable to consume a different key name in *db_transaction* function. Here is the patch