add "shoot yourself in the foot mode" (send arbitrary json)

[yargevad]

This will allow use of every API feature, specifically ones that don't have explicit support in the client library yet.

[kakysha]

Isn't it already implemented in standard library ?)

[yargevad]

Yes, net/http would let you do this. The goal here is that if current users of gosparkpost would like to use new features that the library doesn't yet support, then they don't have to switch everything over to net/http