Froodian fix data race

[froodian]

If you add the new test to the code on master and run go test ./... -race -count=1 -timeout=1m -run . you'll see it fail with

WARNING: DATA RACE
Write at 0x00c0000dc380 by goroutine 17:
  github.com/diegobernardes/ttlcache.(*Cache).SetWithTTL()
      /Users/ian/go/src/github.com/diegobernardes/ttlcache/cache.go:140 +0xa0
  github.com/diegobernardes/ttlcache.TestCache_SkipTtlExtensionOnHit_ForRacesAcrossGoroutines.func1.1()
      /Users/ian/go/src/github.com/diegobernardes/ttlcache/cache.go:131 +0x7d

Previous read at 0x00c0000dc380 by goroutine 16:
  github.com/diegobernardes/ttlcache.TestCache_SkipTtlExtensionOnHit_ForRacesAcrossGoroutines.func2.1()
      /Users/ian/go/src/github.com/diegobernardes/ttlcache/cache.go:172 +0x94

this resolves that.

[ReneKroon]

Thanks for reporting.