Enable number_to_percentage to keep the number's precision by allowing :precision option value to be nil

When converting numbers to percentage using number_to_percentage helper method, it is sometimes necessary to keep the numbers’ precision as is, without having to set an arbitrary precision value.

for example, we have an array of numbers, and I would like to keep their precision so that 

```ruby
numbers = [1000, 0.12, 43.1]
numbers.map {|x| number_to_percentage(x, precision: nil)}  # =>  ['1000%', '0.12%', '43.1%']
```