Adds ActiveHash#find_by functionality

Functionality similar to ActiveRecord::FinderMethods#find_by (https://github.com/rails/rails/blob/a8827cb9afdf122c003fbff1bfcc28bda2994d04/activerecord/lib/active_record/relation/finder_methods.rb#L82).

Includes updated specs.
1.9 hash syntax is why this is failing CI on 1.8.7.