association_matcher foreign_key refinement

#66

has_many and has_one tests failed with the reverse association has a
nonstandard name, (eg "belongs_to :author, :class_name => :User)

corrected by having have_one and have_many association tests check for an
:inverse_of on their association, and use the inverse relationship's
foreign key info if it can be found

lib/shoulda/matchers/active_record/association_matcher.rb

@@ -240,7 +240,15 @@ def associated_class
         end
 
         def foreign_key
-          reflection.respond_to?(:foreign_key) ? reflection.foreign_key : reflection.primary_key_name
+          fk_reflection = reflection
+          if [:has_one, :has_many].include?(@macro) && reflection.options.include?(:inverse_of)
+            fk_reflection = associated_class.reflect_on_association(
+              reflection.options[:inverse_of]
+            )
+          end
+          fk_reflection.respond_to?(:foreign_key) ?
+            fk_reflection.foreign_key :
+            fk_reflection.primary_key_name
         end
 
         def through?