Update Date.php preProcessIndex to use format from config

[granitibrahimi]

When a format is specified for a Date field and the field is present on the list, it always throw a DateTime::__construct(): Failed to parse time string exception. Example I have created a date field with the format: "y-m" and when I add that field to the list then this is the thrown exception:

Could not parse '20-10': DateTime::__construct(): Failed to parse time string (20-10) at position 0 (2): Unexpected character {"userId":2,"exception":"[object] (Carbon\\Exceptions\\InvalidFormatException(code: 0): Could not parse '20-10': DateTime::__construct(): Failed to parse time string (20-10) at position 0 (2): Unexpected character at /var/www/html/vendor/nesbot/carbon/src/Carbon/Traits/Creator.php:190) [stacktrace]
#0 /var/www/html/vendor/nesbot/carbon/src/Carbon/Traits/Creator.php(216): Carbon\\Carbon::rawParse('20-10', NULL)
#1 /var/www/html/vendor/statamic/cms/src/Fieldtypes/Date.php(300): Carbon\\Carbon::parse('20-10')
#2 /var/www/html/vendor/statamic/cms/src/Fieldtypes/Date.php(126): Statamic\\Fieldtypes\\Date->parseSaved('20-10')
#3 /var/www/html/vendor/statamic/cms/src/Fieldtypes/Date.php(110): Statamic\\Fieldtypes\\Date->preProcessSingle('20-10')
#4 /var/www/html/vendor/statamic/cms/src/Fields/Field.php(314): Statamic\\Fieldtypes\\Date->preProcess('20-10')
#5 /var/www/html/vendor/laravel/framework/src/Illuminate/Collections/HigherOrderCollectionProxy.php(60): Statamic\\Fields\\Field->preProcess()
#6 [internal function]: Illuminate\\Support\\HigherOrderCollectionProxy->Illuminate\\Support\\{closure}(Object(Statamic\\Fields\\Field), 'date

[edalzell]

Thanks for this @granitibrahimi can you please fix the style issues found by StyleCI?

[granitibrahimi]

Thanks for this @granitibrahimi can you please fix the style issues found by StyleCI?

Hey @edalzell just did it, and please squash the commits since obviously I messed them up lol

[jasonvarga]

Nice find. I've just pushed up a change to use an existing method that already exists for this purpose.

Thanks!