Make own copy of DEFAULT_OPTIONS.middlewares. Close #288

[lolgesten]

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[codecov-io]

Codecov Report

Merging #289 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
+ Coverage    94.6%   94.61%   +0.01%     
==========================================
  Files           6        6              
  Lines         389      390       +1     
==========================================
+ Hits          368      369       +1     
  Misses         21       21

Impacted Files	Coverage Δ
lib/manager.js	94.85% <100%> (+0.03%)	⬆️

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