Correctly abort middleware execution

[9seconds]

There is a use case: you start to process a request but some middleware decides to abort an execution (for example, if you have some caching whatsoever). In that case middlewares can do nothing with response headers.

[codecov-io]

Codecov Report

Merging #18 (2456d76) into master (2b77eb0) will decrease coverage by 0.18%.
The diff coverage is 56.00%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
- Coverage   82.58%   82.39%   -0.19%     
==========================================
  Files          48       48              
  Lines        1424     1426       +2     
==========================================
- Hits         1176     1175       -1     
- Misses        189      192       +3     
  Partials       59       59              

Impacted Files	Coverage Δ
default_layers.go	25.00% <0.00%> (-2.28%)	⬇️
server.go	74.56% <0.00%> (-2.82%)	⬇️
headers/headers.go	81.11% <71.42%> (+0.98%)	⬆️
layers/ctx.go	91.75% <100.00%> (+0.54%)	⬆️
dialers/base.go	65.45% <0.00%> (-5.46%)	⬇️

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