reduce allocations on ActiveView::Digestor.digest method.

Benchmark:

```
Calculating -------------------------------------
   before with cache      6457 i/100ms
    after with cache      6714 i/100ms
-------------------------------------------------
   before with cache    96984.0 (±7.2%) i/s -     484275 in   5.019002s
    after with cache   103156.9 (±8.5%) i/s -     516978 in   5.047322s

Comparison:
    after with cache:   103156.9 i/s
   before with cache:    96984.0 i/s - 1.06x slower
```