Unmarshaling a the string "null" results in a nil which is
unexpected by statecouchdb.  This causes a downstream failure.

This change adds handling for the special case of string "null".

Change-Id: I37cd5c07a6da42a37b441225e809de8c9f0c1d93
Signed-off-by: Chris Elder <chris.elder@us.ibm.com>