Set IDENTITY_INSERT for insert.values({column: expr})

Set IDENTITY_INSERT for insert.values({column: expr})

Fixed bug where the SQL Server "IDENTITY_INSERT" logic that allows an INSERT
to proceed with an explicit value on an IDENTITY column was not detecting
the case where :meth:`.Insert.values` were used with a dictionary that
contained a :class:`.Column` as key and a SQL expression as a value.

Fixes: #4499
Change-Id: Ia61cd6524b030b40a665db9c20771f0c5aa5fcd7