Infinite Scroll: PHP notice, array to string, on customizer save

During testing of another PR today, I noticed PHP notices were showing up with `Array to string conversion` when I saved my widget. 🤔

So, I went to line `1031` in the `formatting.php` and dropped the following at the top of the `wp_check_invalid_utf8()` function:

```php
if ( is_array( $string ) ) {
error_log( wp_debug_backtrace_summary();
}
```

Which gave me the following:

```
[17-Jan-2018 15:26:35 UTC] require('wp-blog-header.php'), require_once('wp-includes/template-loader.php'), do_action('template_redirect'), WP_Hook->do_action, WP_Hook->apply_filters, The_Neverending_Home_Page->ajax_response, do_action('custom_ajax_infinite_scroll'), WP_Hook->do_action, WP_Hook->apply_filters, The_Neverending_Home_Page->query, wp_footer, do_action('wp_footer'), WP_Hook->do_action, WP_Hook->apply_filters, The_Neverending_Home_Page->action_wp_footer_settings, The_Neverending_Home_Page->get_request_path, array_map, sanitize_text_field, _sanitize_text_fields, wp_check_invalid_utf8
[17-Jan-2018 15:26:35 UTC] PHP Notice: Array to string conversion in /srv/users/serverpilot/apps/ericbinnion/public/wp-includes/formatting.php on line 1035
```

When I turned off infinite scroll, I no longer had the PHP notices.

Start in wp-includes/formatting.php at wp_check_invalid_utf8(), then trace the custom_ajax_infinite_scroll path through The_NeverEnding_Home_Page->get_request_path and sanitize_text_field. Reproduce the widget save with Infinite Scroll enabled and confirm the Array to string conversion notice no longer appears while the feature remains enabled.

Written by the indexing model from the issue text.