fix: definition error

[whxaxes]

Checklist

• [x] npm test passes
• [x] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [x] commit message follows commit guidelines

Affected core subsystem(s)

Description of change

fix : https://github.com/node-modules/agentkeepalive/issues/76

[whxaxes]

Unittest for ts has passed

[ngot]

Since Github Actions added in this PR, Travis and Appveyor can be removed I think. The badge in readme.md also needs to be updated.

[fengmk2]

4.1.1