Validation issues with scheming multiple text

Hi there,

I am updating an old CKAN system, that has been heavily customized. I am in the course of updating ckanext-scheming to version 3.0. Previously an older version was used with ckanext-repeating.

We're using repeating text in our dataset schema twice. Here is the author field's section:
```json
{
"default_value": "context_fullname_email",
"display_snippet": "eaw_repeating_text.html",
"modal_text": "author_help",
"form_snippet": "eaw_schema_repeating_text_js.html",
"label": "Author",
"output_validators": "repeating_text_output",
"validators": "scheming_multiple_text eaw_schema_json_not_empty",
"form_placeholder": "Bach, Johann ",
"field_name": "author",
"display_property": "dc:creator",
"form_blanks": 3,
"preset": "multiple_text",
"required": true

}
```

To migrate from ckanext-repeating to the latest version of scheming I changed the validators from **repeating_text** to **scheming_multiple_text** and I added the **preset: multiple_text**. Unfortunately I couldn't find any more information on how to do the migration anywhere.

Now once I test this schema, I have issue submitting the "new dataset form". Somehow the authors I enter into the repeating text fields are not recognized, as I get a missing author warning.

This is caused by the **scheming_multiple_text** validator of ckanext-scheming. Visualizing the validators **data** argument I found that the repeating text fields we use end up in a spot than expected. The repeating text fields end up in **__extras**. But the validator is checking for author.

```python
{('embargo',): '',
('geographic_name',): '',
('has_part',): '',
('id_external',): '',
('is_part_of',): '',
('maintainer',): 'ckadm',
('name',): 'ghfg',
('notes',): 'fdg',
('notes-2',): '',
('open_data',): 'true',
('owner_org',): '87689fbc-3142-40de-89ab-82c760bb34f3',
('private',): 'False',
('publicationlink',): '',
('review_level',): 'none',
('reviewed_by',): '',
('save',): '',
('spatial',): '{}',
('status',): 'incomplete',
('substances',): '',
('substances_generic',): '',
('systems',): '',
('tags_string',): 'federal',
('taxa',): '',
('taxa_generic',): '',
('title',): 'ghfg',
('usage_contact',): 'ckadm',
('variables',): ['alkalinity', ''],
('state',): 'draft',
('type',): 'dataset',
('__extras',): {'_ckan_phase': 'dataset_new_1',
'author-1': 'foerster, christian ""',
'author-2': '',
'author-3': '',
'pkg_name': '',
'timerange-1': '*',
'timerange-2': ''},
('id',): '',
('author',): '',
('author_email',): '',
('maintainer_email',): '',
('license_id',): '',
('url',): '',
('version',): '',
('log_message',): '',
('tag_string',): '',
('return_to',): '',
('separator_domain_metadata',): '',
('ispublication',): '',
('islongterm',): '',
('timerange',): ''}
```

I am not sure how the data is passed and why it ends up in **__extras** instead of author. Does anyone have some advice on how to correct this?

Cheers!