Patch for ActiveRecord::Base.find mixed array of Strings and Symbols for :joins option

added support for mixed arrays of manueal joins and model associations
in ActiveRecord::Base.find's :joins option

Right now it is only possible to eather supply Strings with handcoded
sql join options or an Array/Hash with References to model
associations. But it is not possible for example to say :joins =>
[:person, "ratings inner join post on ratings.post_id = posts.id"]

Added patch and tests ..
https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/3121-activerecordbasefind-mixed-array-of-strings-and-symbols-for-joins-option