Fix response objects not referencing parents correctly

Fixes #103

test/test_response.capnp

@@ -0,0 +1,13 @@
+@0x84249be5c3bff005;
+
+interface Foo {
+  foo @0 () -> (val :UInt32);
+}
+
+struct Bar {
+  foo @0 :Foo;
+}
+
+interface Baz {
+  grault @0 () -> (bar: Bar);
+}

test/test_response.py

@@ -0,0 +1,40 @@
+import pytest
+import capnp
+import os
+import time
+
+import test_response_capnp
+
+class FooServer(test_response_capnp.Foo.Server):
+    def __init__(self, val=1):
+        self.val = val
+
+    def foo(self, **kwargs):
+        return 1
+
+class BazServer(test_response_capnp.Baz.Server):
+    def __init__(self, val=1):
+        self.val = val
+
+    def grault(self, **kwargs):
+        return {"foo": FooServer()}
+
+def test_response_reference():
+    baz = test_response_capnp.Baz._new_client(BazServer())
+
+    bar = baz.grault().wait().bar
+
+    foo = bar.foo
+    # This used to cause an exception about invalid pointers because the response got garbage collected
+    foo.foo().wait()
+
+def test_response_reference2():
+    baz = test_response_capnp.Baz._new_client(BazServer())
+
+    bar = baz.grault().wait().bar
+
+    # This always worked since it saved the intermediate response object
+    response = baz.grault().wait()
+    bar = response.bar
+    foo = bar.foo
+    foo.foo().wait()