fix: spwan pool on request when use agent

[climba03003]

Fixes #206

Agent is actually using origin to determine the Pool that should be used. We do not need to spawn it ourselves.

https://github.com/nodejs/undici/blob/30bea0662a46212be1e4b6d81b37c22858ab8142/lib/agent.js#L107-L122

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[climba03003]

cc @artur-ma

[coveralls]

Pull Request Test Coverage Report for Build 1383920697

• 0 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-2.7%) to 94.605%

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Totals
Change from base Build 1379979487:	-2.7%
Covered Lines:	276
Relevant Lines:	290

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1384390991

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 97.297%

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Totals
Change from base Build 1379979487:	-0.01%
Covered Lines:	285
Relevant Lines:	289

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💛 - Coveralls

[mcollina]

lgtm