added postcondition to ensure the body string set to the response, is the same reference

this is important, since sometime we just do
  rep.set_body (s)
  s.append_string ("..")

library/server/wsf/src/response/wsf_page_response.e

@@ -60,6 +60,8 @@ feature -- Element change
 	set_body (a_body: like body)
 		do
 			body := a_body
+		ensure
+			body_set: a_body /= Void implies body = a_body
 		end
 
 	put_string (a_string: READABLE_STRING_8)