fix: restore auto direct connection behavior

Author: emadum

lib/core/uri_parser.js

@@ -687,6 +687,15 @@ function parseConnectionString(uri, options, callback) {
     return callback(new MongoParseError('directConnection option requires exactly one host'));
   }
 
+  // NOTE: this behavior will go away in v4.0, we will always auto discover there
+  if (
+    parsedOptions.directConnection == null &&
+    hosts.length === 1 &&
+    parsedOptions.replicaSet == null
+  ) {
+    parsedOptions.directConnection = true;
+  }
+
   const result = {
     hosts: hosts,
     auth: auth.db || auth.username ? auth : null,