Attachments stored in the S3 blob store are truncated when a page is deleted or exported as XAR

Additional details on the scope:

• All the code paths that serialize an attachment content through XAROutputFilterStream are affected: the page recycle bin (with the default file store and with the hibernate store), the attachment recycle bin when it uses the hibernate content store (xwiki.store.attachment.recyclebin.content.hint=hibernate, HibernateDeletedAttachmentContent), the XAR export, and XWikiDocument#toXML / XWikiAttachment#toXML with the attachment content. Deleted attachments are not affected with the default file attachment recycle bin content store, which copies the content to the end of the stream.
• Only attachments whose content stream returns a short read are truncated: in practice the attachments larger than a few KB stored in the S3 blob store (the 4096-byte attachments of the test were intact).
• The fix cannot repair what was already written: recycle bin entries created before the fix keep their truncated attachments (the attachment content of a deleted page is removed with the page), so restoring such an entry after upgrading still gives truncated attachments.