Update FetchHttpClient to make fetch function optional.

[dcr-stripe]

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Summary

Updates the FetchHttpClient to make the fetch function optional, defaulting to the global if not provided.

This also purposefully avoids calling this._fetchFn() directly and instead storing it and calling it via the variable. This ensures we call the passed in function with the global scope instead of accidentally calling it with the FetchHttpClient as this. Calling this._fetchFn() caused issues on Cloudflare Workers as it would invoke fetch in the wrong scope, resulting in code failing with:

A hanging Promise was canceled. This happens when the worker runtime is waiting for a Promise from JavaScript to resolve, but has detected that the Promise cannot possibly ever resolve because all code and events related to the Promise's request context have already finished.

This is likely because the global fetch stores state to track pending work, and calling it within a different scope resulted in the work not being tracked.

Examples

Default global fetch

new FetchHttpClient();

Explicitly passing in global fetch

new FetchHttpClient(fetch);

Passing in a custom fetch (eg. node-fetch)

const fetch = require('node-fetch');
new FetchHttpClient(fetch);