restic-server: Fix content length for HEAD requests
restic-server: Fix content length for HEAD requests

bchapuis/restic,ckemper67/restic,middelink/restic,byo/restic,jayme-github/restic,byo/restic,arithmetric/restic,jayme-github/restic,middelink/restic,kurin/restic,fawick/restic,ar-jan/restic,restic/restic,fawick/restic,restic/restic,mappu/restic,ar-jan/restic,stuertz/restic,stuertz/restic,kurin/restic