Fixes uploading of empty files (#25115).

Fixes uploading of empty files (#25115).

- prevents creation of attachment records without existing diskfile and empty
  digest
- adds test case to check file upload API response
- also removes the file size check in ActsAsAttachable which still prevented
  attachment of zero size attachments to containers but only for clients
  without Javascript (where save_attachments is called with the actual file
  upload).

Patch by Jens Kraemer.

git-svn-id: http://svn.redmine.org/redmine/trunk@16341 e93f8b46-1217-0410-a6f0-8f06a7374b81