Check if using a method from a polyfill is the right approach

[cmahnke]

See:

mirador/src/state/sagas/iiif.js

Line 4 in d61b13a

import fetch from 'isomorphic-unfetch';

I'm not a JS programmer and don't really know about the implications, but this line (importing the fetch method from the package isomorphic-unfetch) seems quite strange:

• fetch is a method provided by modern browsers, and the package seems to be a workaround (aka polyfill), so there should be no need to use it directly
• In src/polyfills.js the package unfetch is used, it's a gut feeling and certainly true with other languages, but using two packages providing one functionality is never a good idea, it creates confusion and weird problems.
• The later is the reason why I found it: I Chrome I got the following error message TypeError: Failed to execute 'fetch' on 'Window': Illegal invocation an Firefox also displayed an error regarding problems the Window interface. there are several reports over the net from different packages. Some are related to Webpack, but since i'm using ESBuild, these don't apply to my situation.

Anyways, it works if I just comment the line out, I haven't tested on older browsers yet, but my assumption is, that the polyfill covers this situation...