Can roundrip with type converter and no default ctor

When using the roundtrip traversal strategy, there's a check that the
types involved all have default ctors (otherwise they can't be
deserialized). But this check didn't take into account custom type
converters, which can deserialize without default ctors.
Merge pull request #19 from roji/roundtrip_ctor

Can roundrip with type converter and no default ctor