Fix & test for downloading by asset ID URL

[jwodder]

Fixes #557.

[codecov]

Codecov Report

Merging #561 (a337060) into master (3ded813) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #561      +/-   ##
==========================================
+ Coverage   83.51%   83.63%   +0.12%     
==========================================
  Files          62       62              
  Lines        6489     6500      +11     
==========================================
+ Hits         5419     5436      +17     
+ Misses       1070     1064       -6     

Flag	Coverage Δ
unittests	83.63% <100.00%> (+0.12%)	⬆️

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Impacted Files	Coverage Δ
dandi/download.py	88.09% <100.00%> (+1.64%)	⬆️
dandi/tests/test_download.py	100.00% <100.00%> (ø)
dandi/dandiarchive.py	85.31% <0.00%> (+0.56%)	⬆️

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[yarikoptic]

Thank you @jwodder ! I think I am now getting to what you meant by different returned records for assets - we might need to harmonize them on the client side, may be as part of the #249 (I hope to get back to that one soon, time to progress it forward)