lib: improve the usage of TypeError[INVALID_ARG_TYPE]

The initials of expected in TypeError[ERR_INVALID_ARG_TYPE]
are inconsistent. This change is to unify them.

PR-URL: #16401
Fixes: #16383
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

Author: starkwang

lib/_http_client.js

@@ -105,7 +105,7 @@ function ClientRequest(options, cb) {
     // when createConnection is provided.
   } else if (typeof agent.addRequest !== 'function') {
     throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'Agent option',
-                               ['Agent-like object', 'undefined', 'false']);
+                               ['Agent-like Object', 'undefined', 'false']);
   }
   this.agent = agent;
 

lib/_http_outgoing.js

@@ -652,7 +652,7 @@ function write_(msg, chunk, encoding, callback, fromEnd) {
 
   if (!fromEnd && typeof chunk !== 'string' && !(chunk instanceof Buffer)) {
     throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'first argument',
-                               ['string', 'buffer']);
+                               ['string', 'Buffer']);
   }
 
 
@@ -750,7 +750,7 @@ OutgoingMessage.prototype.end = function end(chunk, encoding, callback) {
   if (chunk) {
     if (typeof chunk !== 'string' && !(chunk instanceof Buffer)) {
       throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'first argument',
-                                 ['string', 'buffer']);
+                                 ['string', 'Buffer']);
     }
     if (!this._header) {
       if (typeof chunk === 'string')