Use :foreign_key name for belongs_to association if specified

Signed-off-by: Justin French <justin@indent.com.au>

lib/formtastic.rb

@@ -1090,6 +1090,7 @@ def create_boolean_collection(options)
     # be used for the input
     #
     #   belongs_to :author; f.input :author; will generate 'author_id'
+    #   belongs_to :entity, :foreign_key = :owner_id; f.input :author; will generate 'owner_id'
     #   has_many :authors; f.input :authors; will generate 'author_ids'
     #   has_and_belongs_to_many will act like has_many
     #
@@ -1098,7 +1099,7 @@ def generate_association_input_name(method)
         if [:has_and_belongs_to_many, :has_many].include?(reflection.macro)
           "#{method.to_s.singularize}_ids"
         else
-          "#{method}_id"
+          reflection.options[:foreign_key] || "#{method}_id"
         end
       else
         method