Fix issue with body json decoder

[vminkobin]

json.NewEncoder adds "\n" at the end of encoded data https://github.com/golang/go/blob/26b48442569102226baba1d9b4a83aaee3d06611/src/encoding/json/stream.go#L196

This can cause potential issues when requests should be signed. Consider this code,

req := httpClient.InitClient("https://test.com", nil)

	body := map[string]interface{}{"payload": "data"}
	bbs, _ := json.Marshal(body)
	signature, _ := crypto.HMACSHA256(bbs, "key")

	res, _ := req.
		Execute(context.Background(),
			httpClient.NewReqBuilder().
				Headers(map[string]string{
					"Signature":    string(signature),
					"Content-Type": "application/json",
				}).
				Method(http.MethodPost).
				PathStatic("/test").
				Body(body).
				Build(),
		)

the server will not get valid signature because json.Marshall will return {"payload": "data"} but request body will be {"payload": "data"}\n