#raw_connection proxy to master_connection

[TripWar]

Currently SDP returns nil for #raw_connection of the standard adapter. This makes it incompatible with gems like Upsert (https://github.com/seamusabshere/upsert) due to the fact that Upsert tries to grab the raw_connection so it can skip ActiveRecord when generating queries for the database.

It makes sense to me that the #raw_connection would return the master connection's raw_connection.

The test I wrote is pretty basic, but more than that, the forked gem works with the Upsert gem without any problems.