Fix serializer unittests

Commit d22b2999 broke the serializer unittests with certain
versions of simplejson. This patch removes sort_keys again
and implements a slightly more efficient way of detecting
simplejson functionality. The serializer unittests no longer
use a partially broken mock, but rather a function to convert all
tuples to lists before comparing.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>