fix: mock node fetch with options.body fn

[KhafraDev]

Fixes #1447

No tests but I could theoretically add them in (by calling npm run node:build and then using that file, but seems pretty hacky).

Verify the fix works:

test.mjs:

import * as undici from './index.js' // 5.2.0
import { fetch } from './undici-fetch.js'

const mockAgent = new undici.MockAgent()
mockAgent.disableNetConnect()
undici.setGlobalDispatcher(mockAgent)
const pool = mockAgent.get('https://example.com')

pool.intercept({
  path: '/path',
  method: 'POST',
  body (body) {
    console.log(body)
    return true
  }
}).reply(200, { ok: 1 }).times(2)

const url = new URL('https://example.com/path').href

// undici fetch v5.2.0 from node_modules
await undici.fetch(url, {
  method: 'POST',
  body: JSON.stringify({ foo: 'bar' })
})

// undici fetch v5.2.0 from node 18.2.0 global
await fetch(url, {
  method: 'POST',
  body: JSON.stringify({ foo: 'bar' })
})

npm run build:node && node test.mjs

Output:

{"foo":"bar"}
{"foo":"bar"}
{"foo":"bar"}

The issue of the function being called 3 times is not an issue with this fix (see comment in the issue)

Everything is done in the repo's root directory.

[codecov-commenter]

Codecov Report

Merging #1464 (f78d34d) into main (84c8a02) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1464      +/-   ##
==========================================
- Coverage   94.54%   94.51%   -0.03%     
==========================================
  Files          49       49              
  Lines        4269     4269              
==========================================
- Hits         4036     4035       -1     
- Misses        233      234       +1     

Impacted Files	Coverage Δ
lib/fetch/index.js	81.19% <ø> (-0.04%)	⬇️
lib/mock/mock-agent.js	100.00% <100.00%> (ø)
lib/mock/mock-utils.js	100.00% <100.00%> (ø)
lib/fetch/request.js	85.95% <0.00%> (-0.43%)	⬇️

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[mcollina]

lgtm

This would really benefit from having a test, as it's a major use case of undici. Maybe open an issue?

[mcollina]

cc @ronag

[KhafraDev]

@mcollina added a test in a not so terrible way

[mcollina]

lgtm