The "_specs" array contains column specifications that have the bind
marker name if there is one. That results in
get_partition_key_bind_indices() not being able to look up a column
definition for such columns. Fix the issue by keeping track of the
actual column specifications passed to add() like Cassandra does.
Fixes#2369
(cherry picked from commit a45e656efb4c6478d80e4dfc18de99b94712eeba)