Wrong remainder calculation

`x.modulo(y)` and `x.remainder(y)` should be equal for positive x and y
```
x.remainder(y) means x-y*(x/y).truncate
a%b = a - (a.to_f/b).floor * b
```

```ruby
x=BigDecimal('0.12345e10')
y=BigDecimal('0.23456e-10')

x.modulo y
# => 0.3456e-11

x.remainder y
# => 0.216e-1
```