[BUG] Date field not converted to string for insertion into database on form submission

## Overview

When using a `type: date` field on a BoltForms form configuration and also setting the form to save to the database - it throws an error stating that the _'Object of class DateTime could not be converted to string'_.

## Details

**BoltForms Version: 4.1.13**
Bolt Version:3.3.6
Install type: Composer install
PHP version: 7.0
Web server: Apache/2.4.18

## Reproduction

#### BoltForms Configuration

```yaml
example_form:
database:
contenttype: examples
fields:
title:
type: text
date:
type: date
submit:
type: submit
```

#### Contenttypes Configuration

``` yaml
examples:
name: Examples
singular_name: Example
fields:
title:
type: text
date:
type: date
```

This configuration should give you the error. Any form with a field of `type: date` and that saves to the database of a content type should provide the same error though.

This seems like it was raised and fixed in the past with [#3459](https://github.com/bolt/bolt/issues/3459) but maybe with recent refactoring this has crept back in?