Add option for custom fetch implementation

[jonasgeiler]

As requested in #269, this PR adds the ability to pass a fetch option when creating a ky instance.
This allows to use custom fetch implementations like isomorphic-unfetch or when using SSR in some frameworks (Sapper in my case).

TODO

• Implementation
• Documentation
• Tests

Fixes #269

[jonasgeiler]

Definitely need some help with the tests as I don't have any experience with ava or whatever you're using here

[sindresorhus]

Definitely need some help with the tests as I don't have any experience with ava or whatever you're using here

Give it a try and we'll be happy to help if you get stuck.

https://github.com/avajs/ava/blob/master/docs/01-writing-tests.md

[sholladay]

The tests for this feature should go in here:
https://github.com/sindresorhus/ky/blob/master/test/fetch.js

You can run the tests in that file with a command like this:

npx ava test/fetch.js

Or to run all of the tests:

npx ava

[jonasgeiler]

Just took the other test in test/fetch.js and modified it to use the option instead of the global for the custom fetch function - hope this is enough!
Let me know if you need anything else

[jonasgeiler]

@sholladay @sindresorhus Is this ready to merge?

[sholladay]

LGTM. :)

[jonasgeiler]

@sholladay yup! Ready to merge on my side 🚀

[jonasgeiler]

I think the travis build is broken @sholladay

[sholladay]

Hmm. The logs on Travis indicate that the build completed successfully, but for some reason GitHub shows the build as still running.