fix(postgrest): set columns parameter for single-object insert/upsert

[mandarini]

Summary

• Fix single-object upsert() and insert() to set the columns URL parameter
• Add defaultToNull option to single-object overloads for consistency
• Previously only array (bulk) operations set columns, causing single-object upserts to attempt updating ALL table columns, setting missing ones to null

Closes #1653

[coveralls]

coverage: 80.696% (-0.5%) from 81.173%
when pulling 48c84b8 on fix/single-object-upsert-columns
into 8769da9 on master.

[laurenceisla]

The issue #1653 is similar to the one I opened in PostgREST/postgrest#4046, which was closed as not-planned. This is because, for an upsert, it's natural to provide all of the values in the body to insert-or-update a row. So this problem happens because the issue's OP wants to do a bulk update, which is not implemented yet (see PostgREST/postgrest#1959).

They confirm this in the issue (emphasis mine):

I want to bulk update, but must use upsert. I use upsert but receive issues on non-null columns when they are not provided, despite the value existing on the record in the db.

They're forced to use upsert here, but it's not the right tool for this job. I'm afraid the only fix is to make the bulk update work upstream (in PostgREST).

[mandarini]

I am closing this PR after discussing with @laurenceisla