Add JSON API Accept/Produces

[manveru]

This is required for http://jsonapi.org/format/#content-negotiation

[codecov-io]

Codecov Report

Merging #36 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #36      +/-   ##
=========================================
+ Coverage   94.55%   94.6%   +0.05%     
=========================================
  Files           4       4              
  Lines         367     371       +4     
=========================================
+ Hits          347     351       +4     
  Misses         15      15              
  Partials        5       5

Impacted Files	Coverage Δ
operation.go	100% <100%> (ø)	⬆️

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[easonlin404]

LGTM