Holen's isomorphic-unfetch collide with application's isomorphic-unfetch

[thangngoc89]

I'm developing an isomorphic app with Next.js and React.js. I have isomorphic-unfetch imported globally like this

import "isomorphic-unfetch`;

in every page.

When I import Holen to a page (any page), it will throw an error say that fetch is undefined even on browser that have native fetch.

Removing the import either in Holen's code or in my application's code will "fix" the error.

So I think that Holen should require user to have fetch polyfill globally available instead of importing the polyfill directly.

Thoughts?

[tkh44]

Sure put in a PR.