Make httpless more like nodejs

[cncolder]

Send string error if content type is text. Otherwise send json.

[codecov]

Codecov Report

Merging #19 into master will increase coverage by 1.68%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
+ Coverage   60.54%   62.22%   +1.68%     
==========================================
  Files          14       14              
  Lines         517      548      +31     
  Branches      104      114      +10     
==========================================
+ Hits          313      341      +28     
- Misses        200      203       +3     
  Partials        4        4

Impacted Files	Coverage Δ
httpless/response.ts	93.22% <86.95%> (-4.08%)	⬇️
microless/index.ts	87.69% <92.3%> (+1.97%)	⬆️

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