Add base handler tests

[blink1073]

Caught a bug in AuthenticatedFileHandler.get. When .head is called, it calls .get with include_body=False.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.34%. Comparing base (bd236f1) to head (81f4613).
Report is 275 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1090      +/-   ##
==========================================
+ Coverage   78.64%   79.34%   +0.70%     
==========================================
  Files          68       68              
  Lines        8301     8294       -7     
  Branches     1650     1649       -1     
==========================================
+ Hits         6528     6581      +53     
+ Misses       1347     1285      -62     
- Partials      426      428       +2     

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