import unit test, more use cases for num_databases

Per Jeff's suggestion, adding a unit test for 'use $class num_databases => 21'.

Adding a helpful error message if you try to select() past the number of
databases that you've got configured--should the error message be the same
as redis's "ERR invalid DB index"?

Also expanding it so you can do $class->new(num_databases => 21)--it's not my
use case but I can see where it would come up.