Make METAREFRESH_IGNORE_TAGS an empty list by default

[Gallaecio]

Fixes #3844

[codecov]

Codecov Report

Merging #4311 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4311      +/-   ##
==========================================
+ Coverage   83.93%   84.09%   +0.16%     
==========================================
  Files         166      166              
  Lines        9870     9998     +128     
  Branches     1469     1497      +28     
==========================================
+ Hits         8284     8408     +124     
+ Misses       1333     1331       -2     
- Partials      253      259       +6

Impacted Files	Coverage Δ
scrapy/settings/default_settings.py	98.97% <100%> (+0.26%)	⬆️
scrapy/crawler.py	89.17% <0%> (-0.1%)	⬇️
scrapy/utils/log.py	89.31% <0%> (+0.42%)	⬆️
scrapy/utils/trackref.py	85.71% <0%> (+2.85%)	⬆️
scrapy/utils/reactor.py	87.09% <0%> (+15.22%)	⬆️

[kmike]

Thanks @Gallaecio!