Fix sentry-rails' backtrace_cleanup_callback injection

[st0012]

As spotted in #1076 (comment), sentry-rails always overrides the backtrace_cleanup_callback provided by the user. It should respect the existing value instead.

[codecov-commenter]

Codecov Report

Merging #1510 (60cdc6e) into master (bae8c5b) will increase coverage by 0.56%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1510      +/-   ##
==========================================
+ Coverage   98.22%   98.78%   +0.56%     
==========================================
  Files         218      123      -95     
  Lines       10562     6723    -3839     
==========================================
- Hits        10374     6641    -3733     
+ Misses        188       82     -106     

Impacted Files	Coverage Δ
sentry-rails/lib/sentry/rails/railtie.rb	100.00% <100.00%> (ø)
...rails/spec/sentry/rails/controller_methods_spec.rb	100.00% <100.00%> (ø)
sentry-rails/spec/sentry/rails_spec.rb	100.00% <100.00%> (ø)
...ven/spec/raven/processors/removestacktrace_spec.rb
sentry-raven/lib/raven/breadcrumbs.rb
sentry-raven/spec/support/test_rails_app/app.rb
...y-raven/lib/raven/integrations/rails/active_job.rb
...raven/spec/raven/breadcrumbs/sentry_logger_spec.rb
sentry-raven/spec/raven/processors/cookies_spec.rb
sentry-raven/spec/raven/configuration_spec.rb
... and 88 more

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