Ability to add fallback content-type value for response auto unmarshaling

[jeevatkm]

Implementation of #92

[codecov]

Codecov Report

Merging #95 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
+ Coverage    96.8%   96.83%   +0.02%     
==========================================
  Files           9       10       +1     
  Lines         970      978       +8     
==========================================
+ Hits          939      947       +8     
  Misses         16       16              
  Partials       15       15

Impacted Files	Coverage Δ
request17.go	100% <ø> (ø)	⬆️
request16.go	100% <ø> (ø)	⬆️
middleware.go	92.2% <100%> (ø)	⬆️
request.go	100% <100%> (ø)	⬆️
util.go	100% <100%> (ø)

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