diff --git a/extras/sponge.rb b/extras/sponge.rb
index 24a646bd8..864482bbe 100644
--- a/extras/sponge.rb
+++ b/extras/sponge.rb
@@ -180,7 +180,7 @@ def fetch(url, method = :get, fields = nil, raw_post_data = nil, headers = {}, l
         post_data = raw_post_data
         send_headers["Content-Type"] = "application/x-www-form-urlencoded"
       else
-        post_data = fields.map { |k, v| "#{k}=#{v}" }.join("&")
+        post_data = encode_fields(fields)
       end
 
       send_headers["Content-Length"] = post_data.length.to_s
@@ -263,4 +263,26 @@ def dputs(string)
       puts string
     end
   end
+
+  def encode_fields(fields)
+    fields.map { |k, v|
+      if v.is_a?(Hash)
+        # :user => { :name => "hi", :age => "1" }
+        # becomes
+        # user[hame]=hi and user[age]=1
+        v.map { |vk, vv|
+          [ CGI.escape("#{k}[#{vk}]"), CGI.escape(vv.to_s) ].join("=")
+        }
+      elsif v.is_a?(Array)
+        # :user => [ "one", "two" ]
+        # becomes
+        # user[]=one and user[]=two
+        v.map{|vv|
+          [ CGI.escape("#{k}[]"), CGI.escape(vv.to_s) ].join("=")
+        }
+      else
+        [ CGI.escape(k.to_s), CGI.escape(v.to_s) ].join("=")
+      end
+    }.flatten.join("&")
+  end
 end