Add comments and tests for socket aliases.

lib/request_overrider.js

@@ -96,6 +96,9 @@ function RequestOverrider(req, options, interceptors, remove, cb) {
     return getHeader(req, name)
   }
 
+  // ClientRequest.connection is an alias for ClientRequest.socket
+  // IncomingMessage.connection is an alias for IncomingMessage.socket
+  // The same Socket is shared between the request and response to mimic native behavior.
   req.socket = req.connection = response.socket = response.connection = Socket({
     proto: options.proto,
   })

tests/test_request_overrider.js

@@ -329,6 +329,21 @@ test('request emits socket', t => {
   })
 })
 
+test('socket is shared and aliased correctly', t => {
+  nock('http://example.test')
+    .get('/')
+    .reply()
+
+  const req = http.get('http://example.test')
+
+  req.once('response', function(res) {
+    t.is(req.socket, req.connection)
+    t.is(req.socket, res.socket)
+    t.is(res.socket, res.connection)
+    t.end()
+  })
+})
+
 test('socket emits connect and secureConnect', t => {
   t.plan(3)