Moved content disposition code+workarounds to OCP\Response

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.
Added workaround for Android content disposition

Fixes #5807
Added isUserAgent() method to request

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