normalize localhost to 127.0.0.1

[EndangeredMassa]

When using pretender, I'll sometimes see or write localhost and 127.0.0.1 in the same app. It would be handy if these were normalized to the same value.

Here's a stab at it, but I don't mind if there's a better approach. Feel free to close if this isn't worth the trouble.

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I added a test, updated some existing tests, and made sure yarn test-ci still passes.

[xg-wang]

Hmmm they do not seem to be exactly the same, at least for the tests being modified. I tried window.location.host and they will differentiate the localhost vs 127.0.0.1.

I think a better place is

pretender/src/hosts.ts

Line 24 in ddca216

let host = parseURL(url).host;

where we can normalize. What do you think?