Fix/agent error message

src/node/index.js

@@ -300,6 +300,15 @@ Request.prototype._getFormData = function () {
  */
 
 Request.prototype.agent = function (agent) {
+  const isValidAgent =
+    agent instanceof http.Agent || agent instanceof https.Agent;
+
+  if (agent && typeof agent === 'object' && !isValidAgent) {
+    throw new TypeError(
+      '.agent() expects an http(s).Agent instance. ' +
+        'Did you mean to use .cert() and .key()?'
+    );
+  }
   if (arguments.length === 0) return this._agent;
   this._agent = agent;
   return this;

test/node/basic.js

@@ -276,6 +276,24 @@ describe('[node] request', () => {
     });
   });
 
+  describe('.agent({ cert: "cert", key: "key" })', () => {
+    it('should throw a helpful error for configuration objects passed as agent', () => {
+      try {
+        request.get('https://example.com').agent({
+          cert: 'cert',
+          key: 'key'
+        });
+        assert.fail('Expected .agent() to throw a TypeError');
+      } catch (err) {
+        assert(err instanceof TypeError);
+        assert.strictEqual(
+          err.message,
+          '.agent() expects an http(s).Agent instance. Did you mean to use .cert() and .key()?'
+        );
+      }
+    });
+  });
+
   describe('with a content type other than application/json or text/*', () => {
     it('should still use buffering', () => {
       return request