Also check stringified breadcrumbs key when reducing payload size

[st0012]

Fixes #1757

[codecov-commenter]

Codecov Report

Merging #1758 (3ea5a9a) into master (614a436) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1758      +/-   ##
==========================================
- Coverage   98.45%   98.44%   -0.02%     
==========================================
  Files         144      144              
  Lines        8412     8421       +9     
==========================================
+ Hits         8282     8290       +8     
- Misses        130      131       +1     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/transport.rb	99.10% <100.00%> (+0.02%)	⬆️
sentry-ruby/spec/sentry/transport_spec.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/core_ext/object/deep_dup.rb	94.11% <0.00%> (-5.89%)	⬇️

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