Add support for using keyword arguments with a named struct in an RPC

test/test_capability.capnp

@@ -81,3 +81,11 @@ interface TestTailCaller {
 interface TestPassedCap {
   foo @0 (cap :TestInterface) -> (x: Text);
 }
+
+interface TestStructArg {
+  bar @0 BarParams -> (c: Text);
+}
+struct BarParams {
+  a @0 :Text;
+  b @1 :Int32;
+}

test/test_capability.py

@@ -358,3 +358,15 @@ def test_null_cap():
 
     with pytest.raises(capnp.KjException):
         client.foo().wait()
+
+
+class TestStructArg(capability.TestStructArg.Server):
+    def bar(self, a, b, **kwargs):
+        return a + str(b)
+
+
+def test_struct_args():
+    client = capability.TestStructArg._new_client(TestStructArg())
+    assert client.bar(a='test', b=1).wait().c == 'test1'
+    with pytest.raises(capnp.KjException):
+        assert client.bar('test', 1).wait().c == 'test1'