FIX: PostgreSQLDatabase::selectDatabase() should switch to the database if it exists.

Previous implementation of PostgreSQLDatabase::selectDatabase() just updated internal
registers, and expected connectDatabase() or similar to be called.  This is out of line
with MySQLDatabase's behaviour, and frankly a bit stupid.  FullTextSearch's test system
expected different behaviour from selectDatabase() and so this is needed to fix that.

Since it's making PostgreSQLDatabase match the behaviour of MySQLDatabase, I don't consider
it an API change.