Duplicate Rails.logger before assigning it to the SDK

[st0012]

I discovered this while investigating #2031: If we don't duplicate Rails.logger, configurations made to the SDK's logger will be applied to the Rails logger as well.

For example, in an Rails app:

Sentry.init do |config|
  config.logger.level = Logger::WARN
end

Will make the Rails logger's level to be Logger::WARN too. I think most users wouldn't expect this and will find it confusing.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (bc0f8ce) 83.23% compared to head (b286e98) 83.23%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2086   +/-   ##
=======================================
  Coverage   83.23%   83.23%           
=======================================
  Files         119      119           
  Lines        5671     5674    +3     
=======================================
+ Hits         4720     4723    +3     
  Misses        951      951           

Files Changed	Coverage Δ
sentry-rails/lib/sentry/rails/configuration.rb	100.00% <100.00%> (ø)
sentry-rails/spec/dummy/test_rails_app/app.rb	92.64% <100.00%> (ø)
sentry-rails/spec/sentry/rails_spec.rb	96.19% <100.00%> (+0.06%)	⬆️

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