cast() types in batch only if type_affinity is different

cast() types in batch only if type_affinity is different

Batch mode will not use CAST() to copy data if type_ is given, however
the basic type affinity matches that of the existing type.  This to
avoid SQLite's CAST of TIMESTAMP which results in truncation of the
data, in those cases where the user needs to add redundant type_ for
other reasons.

Change-Id: I20f7b399cd3cd7740d67ff7d624aa1da874ebc71
Fixes: #391