diff --git a/lib/net/http/header.rb b/lib/net/http/header.rb
index 7e46e04..2a683b2 100644
--- a/lib/net/http/header.rb
+++ b/lib/net/http/header.rb
@@ -365,7 +365,11 @@ module Net::HTTPHeader
   #    http.set_form_data({"q" => "ruby", "lang" => "en"}, ';')
   #
   def set_form_data(params, sep = '&')
-    query = URI.encode_www_form(params)
+    if String === params
+      query = params
+    else
+      query = URI.encode_www_form(params)
+    end
     query.gsub!(/&/, sep) if sep != '&'
     self.body = query
     self.content_type = 'application/x-www-form-urlencoded'