GET requests have data as string "undefined"

[hogpilot]

I'm running into an issue where a GET request is failing because the axios.request(config) is being called with an invalid data property:

// config object
{
  ...
  "method": "GET",
  "data": "undefined",
  ...
}

This is due to config.data being set here to String(init.body) when init.body === undefined.

I'll submit a PR for review.