Fix handling of oauth_provider argument to panel.io.server.serve

[philippjfr]

Fixes #2629

[codecov]

Codecov Report

Merging #3293 (ee4cdaa) into master (51b8950) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3293      +/-   ##
==========================================
- Coverage   83.34%   83.33%   -0.01%     
==========================================
  Files         195      195              
  Lines       26373    26367       -6     
==========================================
- Hits        21981    21974       -7     
- Misses       4392     4393       +1     

Impacted Files	Coverage Δ
panel/io/server.py	78.36% <0.00%> (-0.05%)	⬇️
panel/tests/command/test_serve.py	83.54% <0.00%> (-1.46%)	⬇️
panel/config.py	59.65% <0.00%> (-0.58%)	⬇️
panel/io/state.py	69.11% <0.00%> (-0.23%)	⬇️
panel/pane/idom.py	27.40% <0.00%> (+0.20%)	⬆️
panel/tests/pane/test_plotly.py	98.77% <0.00%> (+0.61%)	⬆️

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