Fix copy_to overwrite flag

[liudvikasakelis]

Fixes #1535.
While the report mentions duckdb, I chose to write test in SQLite due to there being no other duckdb tests yet. Issue seems backend agnostic anyway.
Will be grateful for reviews :)

[liudvikasakelis]

After some test wrangling, I think this is ready for review. In summary:

• proposed test case I think captures the gist of the bug, even if in SQLite. This case fails without the fix and passes with it.
• reprex provided in the issue copy_to(overwrite=TRUE) not working in duckdb and dbplyr #1535 succeeds without error with provided fix (tested manually)
• errors seen in GH automated checks:
  • test-tbl-sql.R:33:3 - already present in commit 5726930 (tested locally). It's a simple fix but I'm unsure if this PR is the best place 🤷
  • test-backend-postgres.R:341:3 - already present in commit 5726930 (tested locally)
  • test-backend-postgres.R:367:3 - already present in commit 5726930 (tested locally)
  • test-coverage 'has been automatically failed' due to deprecated version

i.e. the fix addresses the issue and doesn't introduce more bugs, to the best of my knowledge