Option for no extra fields when fields_set = "custom"

[steffilazerte]

Option to leave 'fields' NULL (i.e. use just the basic fields) when using a custom fields set in nc_data_dl()

[steffilazerte]

@denislepage, right now when the fields_set = "custom" and the fields supplied are NULL, the API returns all fields (or a lot at least).

Would it be possible (and desirable), if the fields were set to custom and none were supplied to return only the basic fields from the data base? i.e., record_id, collection etc. (all the lower case fields)? This would be then the smallest amount possible.

I'm not 100% sure it's useful, so if it's a pain to implement, it's probably best to just leave it.