Fix #1502: AgaviUploadedFile objects are immutable

This was due to recent refactoring; the array didn't get re-initialized.

Technically, this was always sort of broken, but it only started causing errors after `AgaviUploadedFile` refactorings (the variable is an array first, then gets converted, and now is no longer an `ArrayObject`, so it hits the `offsetSet()` case).