Fix payload not init on error case; Fix payload not overwriting primitives (#50)

[betafcc]

On the last fix (#45), the payload is not correctly being set, see the line:

node-red-contrib-graphql/graphql.js

Line 154 in 36374fa

if (!node.msg.payload) node.msg.payload = {};

If the payload is previously set to a string (as noted by #50), it will not work properly

Besides that, the payload was init only for success case, but not for error case, this PR fixes the payload initialization on both cases.