Issue title: `Error: Invalid field name: (not a string)` when subobjects meet {{#each afFieldNames}}
 Issue body: Hi Aldeed,
here is the repo with my quick reproduction of this strange bug. https://github.com/guidouil/autoform_error

The setup to reproduce is a schema with subobjects and using the `autoForm -> each afFieldNames -> afQuickField` way
I know it doesn't make much sense in this setup but I need it so I can customise some other fields in my form.
Thank you for your time and hard work
cheers
 Comments: 
 Comment 0: There were changes to this in 5.0. See the [readme](https://github.com/aldeed/meteor-autoform#affieldnames) and the [changelog](https://github.com/aldeed/meteor-autoform/blob/master/CHANGELOG.md#500)

{'source': 'GithubIssues', 'repo_name_with_owner': 'aldeed/meteor-autoform', 'issue_id': 63422965, 'repo_stars': 1638.0}