SQL: fix type conversion for Timestamp object typed columns (GH9085)
Closes #9085

@artemyk should only be a very small change
 Maybe you should add `datetime64` to `_SQL_TYPES` also, so it works for legacy mode?
 In fallback mode, `datetime64` is already converted to `datetime` (https://github.com/pydata/pandas/blob/v0.15.2/pandas/io/sql.py#L1351), so that should be ok 
 Oh right, that makes sense!
