fix/single-column-updates

[MiguelMJ]

Currently, all updates follow the format:

UPDATE table SET (columnA, columnB) = (?, ?)

but single column updates like this one:

UPDATE table SET (columnA) = (?)

raise SQL syntax errors.

Example from PostgreSQL:

ERROR: source for a multiple-column UPDATE item must be a sub-SELECT or ROW() expression

So I added a check to avoid parenthesis in single column updates.