tables.values is broken

Reproduces on devel, but not on 1.0.4
```nim
import tables

let t = newTable[int, int]()
t[15] = 1
t[19] = 2
t[17] = 3
t[150] = 4

assert(t.len == 4) # ok

t.del(150)

assert(t.len == 3) # ok

var s = newSeq[int]()
for v in t.values: s.add(v)

assert(s.len == 3) # FAIL!
```