fix test and add deprecation warning

Author: emadum

lib/core/uri_parser.js

@@ -693,6 +693,10 @@ function parseConnectionString(uri, options, callback) {
     hosts.length === 1 &&
     parsedOptions.replicaSet == null
   ) {
+    console.warn(
+      'Automatic direct connection to single hosts is deprecated, please provide the `replicaSet` option' +
+        ' or explicitly set the `directConnection` option'
+    );
     parsedOptions.directConnection = true;
   }
 

test/functional/sharding_connection.test.js

@@ -13,7 +13,11 @@ describe('Sharding (Connection)', function() {
   it('Should use sharded topology', {
     metadata: { requires: { topology: 'sharded' } },
     test: function() {
-      const client = this.configuration.newClient({}, { useUnifiedTopology: true });
+      const client = this.configuration.newClient(
+        {},
+        // note: directConnection: false will be the default behavior in 4.0
+        { useUnifiedTopology: true, directConnection: false }
+      );
       return withClient(client, (client, done) => {
         expect(client).to.exist;
         expect(client)