unpickler: fix datetimes within objects

e27a48cd67133d2b7693a213ed82b79fac33c7f1 introduced
a regression by holding references to the outer dict
object, which is not needed.

Add a testcase for datetimes within objects and
fix the unpickler by removing the call to _mkref().

Closes #22

Signed-off-by: David Aguilar <davvid@gmail.com>