pyldb: Make ldb.Message containment testing consistent with indexing

pyldb: Make ldb.Message containment testing consistent with indexing

Previously, containment testing using the 'in' operator was handled by
performing an equality comparison between the chosen object and each of
the message's keys in turn. This behaviour was prone to errors due to
not considering differences in case between otherwise equal elements, as
the indexing operations do.

Containment testing should now be more consistent with the indexing
operations and with the get() method of ldb.Message.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14845
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14848

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(cherry picked from commit 860d8902a9c502d4be83396598cf4a53c80fea69)