fix: Use EnableBuffering to keep request payload around

[bruno-garcia]

Resolves #249

[codecov-io]

Codecov Report

Merging #250 into master will decrease coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #250     +/-   ##
=========================================
- Coverage   89.28%   89.18%   -0.1%     
=========================================
  Files          81       81             
  Lines        2035     2035             
=========================================
- Hits         1817     1815      -2     
- Misses        218      220      +2

Impacted Files	Coverage Δ
src/Sentry.AspNetCore/SentryMiddleware.cs	98.38% <100%> (ø)	⬆️
src/Sentry/Internal/BackgroundWorker.cs	94.26% <0%> (-1.64%)	⬇️

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