Fix MessageManager#createMessage to properly access 'to' & 'cc' properties

[grahhnt]

Allows createMessage to correctly pull 'to' & 'cc' properties from the AP object to follow json-ld spec

Similar change is present elsewhere (like ActivityPubManager#createInboxesFromCC)

From a similar contribution:

JSON-LD allows for arrays to be specified by just their first value

I cannot find the specification at the moment, but the json-ld.org playground (linked from w3.org) shows this

Activity with solo To object (playground) vs Activity with array of a single To value (playground)

This change will allow for implementations that use npm jsonld (like fedify) to correctly interact with Piefed

[BentiGorlich]

Thanks for bringing this to our attention :)

I think this change would work, but we should implement it in all the places and not just adjust one specific one. If you search the project for reads on this property via ['to'] you'll find a lot of those. The to and cc properties are the most prominent arrays that might contain only one value, but there are more.

However if it is too much work for you, then we can close this PR and open an issue, so we can work on it when we have time :)

[grahhnt]

I have gone through and found the common occurrences of 'to' & 'cc' being accessed and replaced them with a new utility class & method, JsonldUtils::getArrayValue

I have also ran php-cs-fixer to resolve that failed test

[BentiGorlich]

This is looking good, thanks for the work. I think there are a lot more occurrances, but I can always expand that later on. I just have 2 style comments :)