Fix serializing multiple dictionary items to Complex QueryString

Author: mythz

src/ServiceStack.Text/QueryStringSerializer.cs

@@ -239,8 +239,12 @@ public static bool FormUrlEncoded(TextWriter writer, string propertyName, object
             var map = obj as IDictionary;
             if (map != null)
             {
+                var i = 0;
                 foreach (var key in map.Keys)
                 {
+                    if (i++ > 0)
+                        writer.Write('&');
+
                     var value = map[key];
                     writer.Write(propertyName);
                     writer.Write('[');

tests/ServiceStack.Text.Tests/UseCases/StripeSerializationTests.cs

@@ -61,12 +61,16 @@ public void Serializes_Customer_Metadata()
             var dto = new CreateStripeCustomer
             {
                 AccountBalance = 100,
-                Metadata = new Dictionary<string, string> { { "order_id", "1234" } },
+                Metadata = new Dictionary<string, string>
+                {
+                    { "order_id", "1234" },
+                    { "ref_id", "456" },
+                },
             };
 
             var qs = QueryStringSerializer.SerializeToString(dto);
             qs.Print();
-            Assert.That(qs, Is.EqualTo("account_balance=100&metadata[order_id]=1234"));
+            Assert.That(qs, Is.EqualTo("account_balance=100&metadata[order_id]=1234&metadata[ref_id]=456"));
         }
 
         [Test]