Return String for Per-Page header too

- standardized with the Total response header
- and also see commit: 03bbc68 and issue #6

Author: marlonandrade

lib/rails/pagination.rb

@@ -41,7 +41,7 @@ def _paginate_collection(collection, options={})
 
       headers['Link']     = links.join(', ') unless links.empty?
       headers['Total']    = ApiPagination.total_from(collection)
-      headers['Per-Page'] = options[:per_page]
+      headers['Per-Page'] = options[:per_page].to_s
 
       return collection
     end