- added isUserAgent() method to OC_Request which makes it possible to
  test it
- OC_Response::setContentDisposition now uses OC_Request::isUserAgent()

Fixes #5807

Added new OC\Response API called setContentDispositionHeader() that
contains the needed workarounds for UTF8 and IE.

Refactored download code to use the new API.

Removed unused trashbin download file.

Conflicts:
	lib/private/vcategories.php