Add IEncoder#reset

[Chase Tingley]

The work in issue #621 made the XMLEncoder stateful, and as a result Jim suggested that we add a reset() method. If we're going to do that, it probably makes sense to do this at the interface level in case it's needed elsewhere.

[Chase Tingley]

Fix issue #633 - Add IEncoder#reset

This is currently no-op for everything except XMLEncoder, JSONEncoder,
and EncoderManager.

→ <<cset 53ec701aca6d>>