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 ="]

Added patch and tests ..