Work around strange webpack behavior

Sometimes webpack will import node-fetch with a default export property, and other times with a commonjs default export.  You never know which one you are gonna get!

src/open-api/index.js

@@ -2,8 +2,8 @@ const get = require('lodash.get')
 const set = require('lodash.set')
 const queryString = require('qs')
 const http = require('http')
-const fetch = require('node-fetch')
-const Headers = fetch.Headers
+const fetch = require('node-fetch').default || require('node-fetch') // Webpack will sometimes export default exports in different places
+const Headers = require('node-fetch').Headers
 const camelCase = require('lodash.camelcase')
 const { JSONHTTPError, TextHTTPError } = require('micro-api-client')
 const debug = require('debug')('netlify:open-api')