Simplify StreamWriter instantiation

Refactor the instantiation of the `StreamWriter` object within a `using` statement by removing unnecessary parameters. The original code included `null` for encoding, `-1` for buffer size, and `true` for leaving the stream open. The updated code uses the default constructor, which only requires the `MemoryStream` object, as the default behavior is sufficient for the intended use case.

generator/ServiceClientGeneratorLib/Generators/Marshallers/JsonRPCRequestMarshaller.tt

@@ -95,7 +95,7 @@ namespace <#=this.Config.Namespace #>.Model.Internal.MarshallTransformations
 #>
             using (MemoryStream memoryStream = new MemoryStream())
             {
-                using (StreamWriter streamWriter = new StreamWriter(memoryStream, null, -1, true))
+                using (StreamWriter streamWriter = new StreamWriter(memoryStream))
                 {
                     JsonWriter writer = new JsonWriter(streamWriter);
 				    writer.Validate = false;