Utilised json.Unmarshal instead of json.NewDecoder

[moredure]

Truth be told, json decoder allocates more, it allocates decoder itself and also have logic/buffers related to stream processing which results in slower single JSON record decoding speed. Also reusing responses might be efficient since all responses quite equal in size here.

[coveralls]

Coverage remained the same at 100.0% when pulling 693555c439f8d6ffe9a27cad1d0f6e1d98cb2043 on moredure:patch-11 into a09d4b5ea2a812939e148b54abb9c2a517aa2411 on sideshow:master.