Fix Response type info, make response 'body' field optional

The `body` field of the Response is optional. Do not error if it is missing.

If the typeinfo of the response. This was incorrectly using the Request type.

Authored by kuafuwang, squashed by ben-clayton.

include/dap/session.h

@@ -201,7 +201,8 @@ class Session {
   virtual void registerHandler(const TypeInfo* typeinfo,
                                const GenericResponseSentHandler& handler) = 0;
 
-  virtual bool send(const dap::TypeInfo* typeinfo,
+  virtual bool send(const dap::TypeInfo* requestTypeInfo,
+                    const dap::TypeInfo* responseTypeInfo,
                     const void* request,
                     const GenericResponseHandler& responseHandler) = 0;
 
@@ -251,9 +252,9 @@ template <typename T, typename>
 future<ResponseOrError<typename T::Response>> Session::send(const T& request) {
   using Response = typename T::Response;
   promise<ResponseOrError<Response>> promise;
-  const TypeInfo* typeinfo = TypeOf<T>::type();
-  auto sent =
-      send(typeinfo, &request, [=](const void* result, const Error* error) {
+  auto sent = send(
+      TypeOf<T>::type(), TypeOf<Response>::type(), &request,
+      [=](const void* result, const Error* error) {
         if (error != nullptr) {
           promise.set_value(ResponseOrError<Response>(*error));
         } else {