http: should support userland Agent

PR-URL: #11567
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/_http_client.js

@@ -89,9 +89,9 @@ function ClientRequest(options, cb) {
     }
     // Explicitly pass through this statement as agent will not be used
     // when createConnection is provided.
-  } else if (!(agent instanceof Agent.Agent)) {
+  } else if (typeof agent.addRequest !== 'function') {
     throw new TypeError(
-      'Agent option must be an instance of http.Agent, undefined or false.'
+      'Agent option must be an Agent-like object, undefined, or false.'
     );
   }
   this.agent = agent;

test/parallel/test-http-client-reject-unexpected-agent.js

@@ -49,7 +49,7 @@ server.listen(0, baseOptions.host, common.mustCall(function() {
   failingAgentOptions.forEach((agent) => {
     assert.throws(
       () => createRequest(agent),
-      /^TypeError: Agent option must be an instance of http.Agent/,
+      /^TypeError: Agent option must be an Agent-like object/,
       `Expected typeof agent: ${typeof agent} to be rejected`
     );
   });