Add ability to use a named route when using sort_link

For example,

sort_link [:engine, Engine::Model.search]

Will use the engine's routing to determine where the route should go. The :engine symbol here should be the name of your engine, i.e. the engine_name setting which generally defaults to the downcased version of your engine's module.

Addresses problem discussed in #84.

lib/ransack/helpers/form_helper.rb

@@ -25,6 +25,12 @@ module Ransack
       end
 
       def sort_link(search, attribute, *args)
+        # Extract out a routing proxy for url_for scoping later
+        if search.is_a?(Array)
+          routing_proxy = search.shift
+          search = search.first
+        end
+
         raise TypeError, "First argument must be a Ransack::Search!" unless Search === search
 
         search_params = params[:q] || {}.with_indifferent_access
@@ -53,9 +59,11 @@ module Ransack
         options.merge!(
           :q => search_params.merge(:s => "#{attr_name} #{new_dir}")
         )
+        options.merge!(:use_route => routing_proxy) if routing_proxy
+
         link_to [ERB::Util.h(name), order_indicator_for(current_dir)].compact.join(' ').html_safe,
-                url_for(options),
+          url_for(options),
-                html_options
+          html_options
       end
 
       private

spec/ransack/helpers/form_helper_spec.rb

@@ -26,7 +26,7 @@ module Ransack
       end
 
       describe '#sort_link' do
-        subject { @controller.view_context.sort_link(Person.search(:sorts => ['name desc']), :name, :controller => 'people') }
+        subject { @controller.view_context.sort_link([:main_app, Person.search(:sorts => ['name desc'])], :name, :controller => 'people') }
 
         it { should match /people\?q%5Bs%5D=name\+asc/}
         it { should match /sort_link desc/}
@@ -35,4 +35,4 @@ module Ransack
 
     end
   end
-end
+end