httphelper: Fix encoding JSONError responses
httphelper: Fix encoding JSONError responses

Calling `json.Marshal` on `JSONError` instead of `*JSONError` causes the
detail field to be base64 encoded.

https://play.golang.org/p/xzPIPOrfoO
https://play.golang.org/p/55jQt2QD_e

Signed-off-by: Jesse Stuart <a5c95b3d7cb4d0ae05a15c79c79ab458dc2c8f9e@jessestuart.ca>

technosophos/flynn,rikur/flynn,flynn/flynn,shads196770/flynn,justintung/flynn,felixrieseberg/flynn,flynn/flynn,pkdevbox/flynn,clifff/flynn,benbjohnson/flynn,technosophos/flynn,justintung/flynn,shads196770/flynn,TribeMedia/flynn,TribeMedia/flynn,supermario/flynn,lmars/flynn,whouses/flynn,ozum/flynn,josephglanville/flynn,justintung/flynn,technosophos/flynn,clifff/flynn,lmars/flynn,clifff/flynn,supermario/flynn,schatt/flynn,kgrz/flynn,rikur/flynn,justintung/flynn,pkdevbox/flynn,TribeMedia/flynn,pkdevbox/flynn,arekkas/flynn,supermario/flynn,flynn/flynn,rikur/flynn,felixrieseberg/flynn,GrimDerp/flynn,whouses/flynn,tonicbupt/flynn,kgrz/flynn,Brandywine2161/flynn,whouses/flynn,philiplb/flynn,clifff/flynn,shads196770/flynn,kgrz/flynn,philiplb/flynn,whouses/flynn,tonicbupt/flynn,TribeMedia/flynn,clifff/flynn,GrimDerp/flynn,arekkas/flynn,whouses/flynn,schatt/flynn,benbjohnson/flynn,josephglanville/flynn,jzila/flynn,felixrieseberg/flynn,Brandywine2161/flynn,shads196770/flynn,supermario/flynn,jzila/flynn,rikur/flynn,jzila/flynn,Brandywine2161/flynn,Brandywine2161/flynn,clifff/flynn,arekkas/flynn,kgrz/flynn,philiplb/flynn,GrimDerp/flynn,flynn/flynn,shads196770/flynn,lmars/flynn,Brandywine2161/flynn,schatt/flynn,tonicbupt/flynn,arekkas/flynn,supermario/flynn,tonicbupt/flynn,schatt/flynn,philiplb/flynn,shads196770/flynn,GrimDerp/flynn,ozum/flynn,supermario/flynn,philiplb/flynn,ozum/flynn,justintung/flynn,whouses/flynn,benbjohnson/flynn,technosophos/flynn,jzila/flynn,flynn/flynn,josephglanville/flynn,ozum/flynn,ozum/flynn,benbjohnson/flynn,rikur/flynn,felixrieseberg/flynn,benbjohnson/flynn,TribeMedia/flynn,lmars/flynn,pkdevbox/flynn,kgrz/flynn,GrimDerp/flynn,jzila/flynn,kgrz/flynn,benbjohnson/flynn,technosophos/flynn,josephglanville/flynn,TribeMedia/flynn,pkdevbox/flynn,lmars/flynn,schatt/flynn,josephglanville/flynn,ozum/flynn,GrimDerp/flynn,Brandywine2161/flynn,pkdevbox/flynn,tonicbupt/flynn,felixrieseberg/flynn,josephglanville/flynn,arekkas/flynn,schatt/flynn,jzila/flynn,rikur/flynn,justintung/flynn