Test for new find_obfuscated method

This method is intended to give the same functionality
of ActiveRecord::Base#find which raises an exception
when a record is not found.  In the current implementation
that functionality is missed, forcing programmers to ask
whether find_by_obfuscated_id returns nil or not.
find_obfuscated implementation
Merge pull request #2 from minostro/master

Implement a new method called find_obfuscated