Tweak implementation of tbl() method

[krlmlr]

For #36.

[codecov-commenter]

Codecov Report

Merging #37 (240ee58) into main (933b747) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 240ee58 differs from pull request most recent head 34e6432. Consider uploading reports for the commit 34e6432 to get more accurate results

@@           Coverage Diff           @@
##             main      #37   +/-   ##
=======================================
  Coverage   85.73%   85.73%           
=======================================
  Files         106      106           
  Lines        3602     3602           
=======================================
  Hits         3088     3088           
  Misses        514      514           

Files	Coverage Δ
R/backend-dbplyr__duckdb_connection.R	92.10% <100.00%> (ø)

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[krlmlr]

Done now.