Trailer headers

[sagebind]

Add new API for accessing response trailer headers.

See #157.

[codecov]

Codecov Report

Merging #256 (135fd99) into master (929fa75) will increase coverage by 0.14%.
The diff coverage is 77.04%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
+ Coverage   74.67%   74.81%   +0.14%     
==========================================
  Files          57       58       +1     
  Lines        3009     3129     +120     
==========================================
+ Hits         2247     2341      +94     
- Misses        762      788      +26     

Impacted Files	Coverage Δ
src/lib.rs	76.19% <ø> (ø)
src/trailer.rs	56.14% <56.14%> (ø)
tests/headers.rs	97.50% <94.44%> (-2.50%)	⬇️
src/handler.rs	72.44% <100.00%> (+1.34%)	⬆️
src/response.rs	60.52% <100.00%> (+2.19%)	⬆️
src/agent/mod.rs	73.17% <0.00%> (+0.48%)	⬆️

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[sagebind]

@teto-bot rustfmt

[sagebind]

@teto-bot rustfmt