Accept data: URLs

[kdau]

Browsers have supported data: URLs in the fetch API for some time now. node-fetch will support them as well in its upcoming v3. The fetch API is the simplest and most reliable way to extract files from data: URLs.

Adding this support precludes a future "data:" matching prefix. This change should not break any existing HTTP(S) matchers, but since the "data:" scheme was previously stripped from URLs passed to matchers, this change will break any existing matchers for data: URLs that were full-string, "begin:", front-anchored regex/glob, or otherwise assumed that the "data:" scheme would be absent.

I did not include support for blob: URLs since they are opaque, less feasible to meaningfully test, and not supported by node-fetch.

[kdau]

To clarify about existing data: URL matchers, here's what mine looks like. It is not front-anchored and so would not break, but it only deals with Base64-encoded data (which is adequate for my use case).

fetchMock.get (/;base64,/, async (url) => nodeFetchActual (`data:${url}`));

[wheresrhys]

Thanks for working on this, and sorry for the very slow response. I'll pull it into fetch-mock@10