Use correct way of getting the database name

`db` is now a proper object.

Author: stevenpost

templates/mongoshrc.js.erb

@@ -34,7 +34,7 @@ if (authRequired()) {
   db.getMongo().setReadPref('primaryPreferred')
   <%- end -%>
   try {
-    var prev_db = db
+    var prev_db = db.getName()
     db = db.getSiblingDB('admin')
     db.auth('<%= @admin_username %>', '<%= @admin_password_unsensitive %>')
     db = db.getSiblingDB(prev_db)