Readme: Not raise error if params not defined in custom config

Author: Paulo Mateus Moura da Silva

README.md

@@ -49,14 +49,14 @@ ApiPagination.configure do |config|
   config.page_param = :page
   # or
   config.page_param do |params|
-    params[:page][:number]
+    params[:page][:number] if params[:page]
   end
 
   # Optional: what parameter should be used to set the per page option
   config.per_page_param = :per_page
   # or
   config.per_page_param do |params|
-    params[:page][:size]
+    params[:page][:size] if params[:page]
   end
 
   # Optional: Include the total and last_page link header