fix: summarise() keeps "duckplyr_df" class

[krlmlr]

Closes #63.

[codecov-commenter]

Codecov Report

Merging #64 (e2e9915) into main (106809f) will increase coverage by 0.09%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head e2e9915 differs from pull request most recent head 01fbabd. Consider uploading reports for the commit 01fbabd to get more accurate results

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@@           Coverage Diff            @@
##            main     #64      +/-   ##
========================================
+ Coverage   7.43%   7.53%   +0.09%     
========================================
  Files        121     121              
  Lines      19115   19120       +5     
========================================
+ Hits        1421    1440      +19     
+ Misses     17694   17680      -14     

Files	Coverage Δ
R/relational.R	90.57% <100.00%> (+0.20%)	⬆️
R/summarise.R	79.59% <100.00%> (+0.86%)	⬆️

... and 8 files with indirect coverage changes

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