The function needs to return a string to be added to the COPY stream, we
still need to make sure whatever given here looks like an integer. Given
the very dynamic nature of data types in SQLite, the integer-to-string
function was already a default now, but failed to be published before in
its fixed version, somehow.
This allows users to benefit from the same flexible machinery when using
SQLite as when using MySQL, and also allows to add some more default
cast rules too.
