Fix AS::NumberHelper results with rationals

[This change](https://github.com/rails/rails/commit/9e997e9039435617b6a844158f5437e97f6bc107) introduced a bug where Rational numbers where rounded always as if the `:significant` option is true, so `:precision` was incorrectly being applied.

Both tests in actionview and activesupport included in this PR would fail with the current code. 

before:
````ruby
ActiveSupport::NumberHelper.number_to_rounded Rational(10, 3), precision: 2  #=> "3.3"
````

after:
````ruby
ActiveSupport::NumberHelper.number_to_rounded Rational(10, 3), precision: 2  #=> "3.33"
````
Not sure if I should modify the changelog.