Improve handling of read types on custom model casts

[tadhgboyle]

• Added or updated tests
• Documented user facing changes (I don't think this is needed?)
• Updated CHANGELOG.md

Changes
This PR fixes a pretty annoying issue while using Larastan and model casts.
For example, when using the Laravel Money package, it has a custom cast class \Cknow\Money\Casts\MoneyIntegerCast.
This casts get method returns a \Cknow\Money\Money object, but Larastan only detected the MoneyIntegerCast as the return type.
This causes dozens of errors similar to:

Parameter #1 $price of static method                              
         `App\Helpers\TaxHelper::calculateFor()` expects `Cknow\Money\Money`
         `Cknow\Money\Casts\MoneyIntegerCast` given.

To fix this issue, I have used reflection to determine the return type of the casts underlying get method, and (if it is defined), use that as the read type for the attribute.

I am aware of this PR #1333, but I thought a smaller PR to fix this problem faster would be nice, until that one gets merged.

[canvural]

Since #1333 is merged, we no longer need this I guess. Please send a new PR if you see that something can be improved. Thanks.