Added setdefault to BaseSettings

[jxlil]

Added the setdefault method to the BaseSettings class.

Now setdefault works as expected, if the key does not exist in BaseSettings then this key will be added and default value will be assigned to it and this value will be returned. For example:

from scrapy.settings import BaseSettings

settings = BaseSettings()
# 'some_key' does not exist in BaseSettings, so it returns 'default_value'
print(settings.setdefault("some_key", "default_value"))

# 'some_key' now gets 'default_value'
print(settings.get("some_key"))

Closes #5811

[codecov]

Codecov Report

Merging #5821 (33eda1c) into master (78eaf06) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 33eda1c differs from pull request most recent head e8ee0d6. Consider uploading reports for the commit e8ee0d6 to get more accurate results

@@           Coverage Diff           @@
##           master    #5821   +/-   ##
=======================================
  Coverage   88.94%   88.95%           
=======================================
  Files         162      162           
  Lines       11002    11007    +5     
  Branches     1798     1799    +1     
=======================================
+ Hits         9786     9791    +5     
  Misses        937      937           
  Partials      279      279           

Impacted Files	Coverage Δ
scrapy/settings/__init__.py	98.24% <100.00%> (+0.05%)	⬆️

[Gallaecio]

Nice!