[FIX] add_fields: Respect sql_type and build safe query
[FIX] add_fields: Respect sql_type and build safe query

- Until now, `sql_type` was being ignored. Fix #171 and respect it.
- Use safe sql builder. If, i.e. one column was called "select", previous implementation with `AsIs()` would fail.

OCA/openupgradelib