Replace StringBuilder.Replace with StringBuilder.Append for building request URL

[paulomorgado]

This PR replaces the use of StringBuilder.Replace in the generation of the request URL with appending URL parts.

Before:

urlBuilder_.Append("sum/{a}/{b}");
urlBuilder_.Replace("{a}", System.Uri.EscapeDataString(ConvertToString(a, System.Globalization.CultureInfo.InvariantCulture)));
urlBuilder_.Replace("{b}", System.Uri.EscapeDataString(ConvertToString(b, System.Globalization.CultureInfo.InvariantCulture)));

After:

urlBuilder_.Append("sum");
urlBuilder_.Append('/');
urlBuilder_.Append(System.Uri.EscapeDataString(ConvertToString(a, System.Globalization.CultureInfo.InvariantCulture)));
urlBuilder_.Append('/');
urlBuilder_.Append(System.Uri.EscapeDataString(ConvertToString(b, System.Globalization.CultureInfo.InvariantCulture)));

This would be easier if the model for generation produced the different path parts already split and classified.