Duplicate original response headers and POST

[mikker]

Fixes #353

[codecov-io]

Codecov Report

Merging #354 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #354      +/-   ##
==========================================
- Coverage   96.77%   96.76%   -0.01%     
==========================================
  Files          78       78              
  Lines        2477     2475       -2     
==========================================
- Hits         2397     2395       -2     
  Misses         80       80

Impacted Files	Coverage Δ
lib/elastic_apm/context_builder.rb	98.03% <100%> (-0.08%)	⬇️
lib/elastic_apm/middleware.rb	97.29% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7f80637...422ecc3. Read the comment docs.