fix: correctly assign username to X509 auth command (#2587)

A typo resulted in a provided username not being properly assigned to the command
send to the server when using X509 authentication.

NODE-2869

Author: through-a-haze

lib/core/auth/x509.js

@@ -26,7 +26,7 @@ class X509 extends AuthProvider {
 function x509AuthenticateCommand(credentials) {
   const command = { authenticate: 1, mechanism: 'MONGODB-X509' };
   if (credentials.username) {
-    Object.apply(command, { user: credentials.username });
+    Object.assign(command, { user: credentials.username });
   }
 
   return command;