Sendable::InitSendable: force builder to be a reference

- Not really sure why it isn't already a reference?
- Fixes #630

Author: virtuald

gen/Sendable.yml

@@ -8,3 +8,8 @@ classes:
     shared_ptr: true
     methods:
       InitSendable:
+        virtual_xform: |
+          [&](py::function fn) {
+            auto builderHandle = py::cast(builder, py::return_value_policy::reference);
+            fn(builderHandle);
+          }