Use attr keys when testing bulk update params for primary key

Use attr keys when testing bulk update params for primary key

Fixed bug in :meth:`.Session.bulk_update_mappings` where alternate mapped
attribute names would result in the primary key column of the UPDATE
statement being included in the SET clause, as well as the WHERE clause;
while usually harmless, for SQL Server this can raise an error due to the
IDENTITY column.  This is a continuation of the same bug that was fixed in
:ticket:`.3849`, where testing was insufficient to catch this additional
flaw.

Fixes: #4357
Change-Id: Iead058c0465dfa31c5b8a8780769278b7000acc8