unpack encoding inconsistent with MRI

### Environment

```
jruby 9.2.7.0 (2.5.3) 2019-04-09 8a269e3 Java HotSpot(TM) 64-Bit Server VM 25.192-b12 on 1.8.0_192-b12 +jit [darwin-x86_64]
```

### Expected Behavior

```ruby
p "str".unpack("H*")[0].encoding
```

returns `Encoding:US-ASCII`.

### Actual Behavior

It returns `Encoding:ASCII-8BIT`.

Edit: Narrowed down the issue to `unpack` instead of `securerandom`.