Setting fields that weren't set before using method missing

Ok, this is the last commit related to setting the custom field values in the Person. This allows the user to use method_missing when setting a field that wasn't retrieved before (e.g. a new person). It was similar as the done via the field method, but in this case we do not know the exact name of the field and thus need to iterate over all to find the one.

Got some other changes coming after this, but not in the same methods anymore.