PR: Update new_defaults dictionary instead of copy

[goanpeca]

Fixes a bug introduced on #188 which broke the overrides mechanism for defaults that ara dictionaries 😞

[codecov-commenter]

Codecov Report

Merging #194 (3fa2d0d) into master (13a0b8a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #194   +/-   ##
=======================================
  Coverage   78.63%   78.63%           
=======================================
  Files          27       27           
  Lines        2017     2017           
=======================================
  Hits         1586     1586           
  Misses        431      431           

Impacted Files	Coverage Δ
jupyterlab_server/settings_handler.py	85.45% <100.00%> (ø)
jupyterlab_server/tests/test_settings_api.py	100.00% <100.00%> (ø)

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[blink1073]

Thanks!