Do not CAST(x as JSON) in SQLite

Do not CAST(x as JSON) in SQLite

Fixed issue where the CAST applied to a JSON column when copying a SQLite
table during batch mode would cause the data to be lost, as SQLite's CAST
with JSON appears to convert the data to the value "0". The CAST is now
skipped in a dialect-specific manner, including for JSON columns on SQLite.
Pull request courtesy Sebastián Ramírez.

Fixes: #697
Closes: #698
Pull-request: https://github.com/sqlalchemy/alembic/pull/698
Pull-request-sha: 6618325258bd90ec257b09c17d44421a5642b1b1

Change-Id: Ia152ea7386e64efb2194aa836dc57754f979e204