Fix serialization of iterable types

So far, we have not supported serialization of iterable type
arguments to RPC methods. However, the recent change to pass
args as a tuple instead of forcing everything to kwargs means
that serialization of non-keyword args is broken. This patch
adds that support, as well as tests for the serializer, which
should have been there in the first place.

Related to blueprint unified-object-model

Change-Id: Ie02fffd89d9b79b4bc8270c14b2860c0e37d7a92
Merge "Fix serialization of iterable types"