zendframework\zend-diactoros is a secondary dependency

[Gary Jones]

Several Zend\Diactoros classes are referenced directly into this repo, even though
zendframework\zend-diactoros is not listed as a dependency.

In this case, it happens to come from another Rareloop package, but it would still be better practice to add zendframework\zend-diactoros to this repo's composer.json, so that a change there doesn't unintentionally break something here.

[Adam Tomat]

That's a good point. There might be a couple more of these kicking around too.

I agree it should be added to this repo's composer.json 👍.

Feel free to PR, or we'll get around to it when we get a chance.