GroupManager#fetch with `omit_members === false` doesn't work

[not-so-smart]

GroupManager#fetch with omit_members === false does not work, I assume it's a problem with the runtime typing and it's expecting an array of members that are not there.

Everything else seems to work okay. Tested it with one id, multiple ids, options with and without pagination, and empty call, it all seems to work

Originally posted by @aidenstern in https://github.com/not-so-smart/node-groupme/issues/52#issuecomment-945307919