fix: ensure cursor readPreference is applied to find operations (#2751)

Author: emadum

lib/operations/find.js

@@ -20,6 +20,9 @@ class FindOperation extends OperationBase {
     // copied from `CommandOperationV2`, to be subclassed in the future
     this.server = server;
 
+    // updates readPreference if setReadPreference was called on the cursor
+    this.readPreference = ReadPreference.resolve(this, this.options);
+
     if (typeof this.cmd.allowDiskUse !== 'undefined' && maxWireVersion(server) < 4) {
       callback(new MongoError('The `allowDiskUse` option is not supported on MongoDB < 3.2'));
       return;