Expose default_db, host, port in Connection

Currently, the default_db attr_reader always returns nil because
the initialize method assigns to a local var instead of @default_db.

Also, @host and @port should probably also be attr_readable.
Merge pull request #2849 from canadaduane/default_db

Expose default_db, host, port in Connection for Ruby driver
merge of #2893 polyglot/regression/2774 fix
Merge branch 'sam_const_func' into next
Merge branch 'next' into sam_2156