SIN should call EncodedData constructor

Creating SINs was broken due to not calling the parent constructor, shich sets
"converts" and "_encoding". I've fixed the problem and added tests that reveal
the error.
set .converters and ._encoding by hand

...revert to previous change, since always calling the constructor of
VersionedData may have unintended consequences. Instead, just set .converts and
._encoding, since they are no longer in the prototype and must be set on the
object itself.
Merge pull request #441 from ryanxcharles/bug/sin-encodeddata

SIN should call VersionedData constructor