Fix URL cache upgrade code
Fix URL cache upgrade code

The upgrade could fail if repeated, which only happens if downloading a
file, running 2.2.4, then downloading the file from <2.2.4, then opening
2.2.4 again.

Also keep etags.

minesense/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,minesense/VisTrails,minesense/VisTrails,minesense/VisTrails,minesense/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails