float format-ing regression in 9.2

```
ruby -e 'n = 97.65625; puts format("%f %f %.1f", n, n.round(1), n)'
```
Ruby 2.3, 2.5 and JRuby 9.1.17.0
```
97.656250 97.700000 97.7
```
JRuby 9.2.5.0
```
97.656250 97.700000 97.6
```
... discovered at *image_optim*'s: https://github.com/toy/image_optim/pull/168