Added toJSON method to Connection class (#849)

* Added toJSON method to Connection class

* Updated test

* Updated typings

test/unit/connection.test.js

@@ -794,3 +794,16 @@ test('Port handling', t => {
 
   t.end()
 })
+
+test('Should not add agent and ssl to the serialized connection', t => {
+  const connection = new Connection({
+    url: new URL('http://localhost:9200')
+  })
+
+  t.strictEqual(
+    JSON.stringify(connection),
+    '{"url":"http://localhost:9200/","id":"http://localhost:9200/","headers":null,"deadCount":0,"resurrectTimeout":0,"_openRequests":0,"status":"alive","roles":{"master":true,"data":true,"ingest":true,"ml":false}}'
+  )
+
+  t.end()
+})