Keep old keys around when not finding kid.

Verified using the following:
(update {:my-url {:kid-1 123}} :my-url merge {:kid-1 999})
=> {:my-url {:kid-1 999}}
(update {:my-url {:kid-1 123}} :my-url merge {:kid-2 999})
=> {:my-url {:kid-1 123, :kid-2 999}}
1 job for master in 31 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #78064
test

00:00:31