fix(ChangeStream): remove startAtOperationTime once we have resumeToken

Removes startAtOperationTime once we have a resumeToken to prevent it from being sent when attempting to resume.

Fixes NODE-2022

Author: timjnh

lib/change_stream.js

@@ -447,6 +447,10 @@ function processNewChange(args) {
 
   changeStream.resumeToken = change._id;
 
+  // wipe the startAtOperationTime if there was one so that there won't be a conflict
+  // between resumeToken and startAtOperationTime if we need to reconnect the cursor
+  changeStream.options.startAtOperationTime = undefined;
+
   // Return the change
   if (eventEmitter) return changeStream.emit('change', change);
   if (typeof callback === 'function') return callback(error, change);