[PR #122] It's not possible to reuse a key after TTL eviction

[ldechoux]

Hello,

I've found an issue when playing with the new TTL feature.
It's not possible to reuse a key to update the cache with a new value.

The sample use case :

c, err := NewCache(...)

// Add a value with key 1 in the cache
c.SetWithTTL(1, 1, 1, 5*time.Second)
_, ok := c.Get(1) // return true

// some time later
// Add another value with key 1 in the cache
c.SetWithTTL(1, 1, 1, 5*time.Second)
_, ok := c.Get(1) // return false

I submit the following PR to correct this issue : #130