[10.x] Fix for sortByDesc ignoring multiple attributes

[TWithers]

When calling sortByDesc() on a collection, it should match the signature of the sortBy() method.

The sortBy() method accepts an array of attributes to sort a collection in 2 different formats:

1. [['attribute1', 'asc|desc'], ['attribute2', 'asc|desc']]
2. ['attribute1', 'attribute2'] which is then transformed to [['attribute1', 'asc'], ['attribute2', 'asc']] in the sortByMany() method.

Essentially, when you pass in an array as the first parameter, you need to be explicit about sorting direction, or it will sort in ascending order. Also, when you pass array as the first parameter, it ignores the $descending flag in the sortBy() method.

Currently, the sortByDesc() method just calls sortBy() and sets the $descending flag to true. So when you pass in an array as the first parameter, if you are not specific, it sorts in ascending order.

This PR resolves this issue by converting the array to the more descriptive format and forcing it to sort in descending order.