see my original issue #196

form-data didn't provide an API for us to detect form-data body contains streamed data or not, which we need to set headers properly. so we had to resort to checking _lengthRetrievers.length.

now with 2.0, this property is renamed and breaks our integration.

ref:

• https://github.com/bitinn/node-fetch/blob/master/index.js#L92-L103