Broken with typescript 4.1.2

[merkys]

node-agent-base fails to build from source with typescript 4.1.2:

$ tsc
src/promisify.ts(27,15): error TS2345: Argument of type 'Duplex | Pick<Agent, "addRequest"> | Agent | undefined' is not assignable to parameter of type 'Duplex | Pick<Agent, "addRequest"> | Agent | PromiseLike<AgentCallbackReturn>'.
  Type 'undefined' is not assignable to type 'Duplex | Pick<Agent, "addRequest"> | Agent | PromiseLike<AgentCallbackReturn>'.

[TooTallNate]

This code in this repository has been moved to the proxy-agents monorepo, so I am closing this pull request. If you feel that this issue still exists as of the latest release, feel free to open a new issue over there.