Ensure Pretender is importable in node

[samselikoff]

Fixes #268

[samselikoff]

@stefanpenner think this is ready for review!

LMK if someone else I should @

[samselikoff]

This works for Pretender's iife build but not it's ES module build, which we found out when trying to use it in a Gatsby app.

The problem is whatwg-fetch is not importable in node.

We're kinda struggling about what to do here. One option would be to vendor whatwg-fetch into a local module and wrap it in a if (typeof window !== undefined). That way that local module (and thus the ES build of Pretender) can be imported in any environment.

What do you think? Any tips on how to solve this in another way?

[samselikoff]

Think we're in a good spot! Gonna test it on some of my projects now

[coveralls]

Coverage increased (+0.04%) to 95.833% when pulling a0a6585 on importable-in-node into 49fdd8a on master.

[samselikoff]

Seems to be working everywhere \o/

[samselikoff]

There's one more thing...

Need more tests to cover ES build case in non-browser environment.

[samselikoff]

To clarify, we need to actually run the real functionality tests against the ES build in a browser-like (jsdom) environment. Need to see what cross-fetch imports there and ensure it lets Pretender work against fetch.

[samselikoff]

Closing for now as stale