oops: Fix download of files with no signature

Files upgraded from versions of osTicket prior to 1.9.1 did not have a
`signature` field in the database. For caching purposes in generating the
Etag HTTP header, the getSignature function cascades through the getKey
method. This may be inconsistent in the signed URL creation.

This patch adds a cascade flag to the getSignature method so the cascading
will not happen unless specifically requested.