fix bug with skin caching that cached capes as non-existent
fix bug with skin caching that cached capes as non-existent

when using cache.save_hash, an `undefined` skin or cape hash means "do not touch"
while `null` means "overwrite as non-existent"

this was sent to redis after calling /capes/Notch for the first time:

  "hmset" "notch" "c" "3f688e0e699b3d9fe448b5bb50a3a288f9c589762b3dae8308842122dcb81" "t" "1429651244222"
  "hmset" "notch" "s" "a116e69a845e227f7ca1fdde8c357c8c821ebd4ba619382ea4a1f87d4ae94" "c" "" "t" "1429651244235"

as you can see, the first request stores the c(ape) but does not touch the s(kin), whereas the second request
sets the s(kin) and replaces the c(ape) value

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