attachment: made download as zip more robust, which now handles:  - missing comment and `unicode` comment  - missing attachment file
attachment: made download as zip more robust, which now handles:
 - missing comment and `unicode` comment
 - missing attachment file

git-svn-id: f68c6b3b1dcd5d00a2560c384475aaef3bc99487@10589 af82e41b-90c4-0310-8c96-b1721e28e2e2

moreati/trac-gitsvn,moreati/trac-gitsvn,moreati/trac-gitsvn,moreati/trac-gitsvn