RowInstance: Fix setting class when there are multiple classes

[janezd]

Issue

Fixes #7291.

Description of changes

Do not ignore the class index.

Note: Failed tests are unrelated. (See #7294, which fixes the problem, though not for oldest supported scikit.)

Includes

• Code changes
• Tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.03%. Comparing base (99bb865) to head (523bc3d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7293   +/-   ##
=======================================
  Coverage   89.02%   89.03%           
=======================================
  Files         336      336           
  Lines       74427    74426    -1     
=======================================
+ Hits        66257    66263    +6     
+ Misses       8170     8163    -7     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.