fix(copy): restore selecting loader for each item

fix(copy): restore selecting loader for each item

In the copy refactoring released in 3.2.11 we changed the behaviour to
choosing a dumper at the first non-null value per column and keeping it.

Turns out more people than expected rely on the more flexible behaviour
of passing strings or types and letting postgres do the conversion.

Restore the previous behaviour; however it's possible to use
`copy.set_types()` to set dumpers per column and gain something in
performance.

Fix #1192