Support `exception:` keyword argument in `Kernel#Rational()`.
If `exception:` is `false`, `Kernel#Rational()` returns `nil` if the given
value cannot be interpreted as a rational value.
The default value of `exception:` is `true`.
This is part of [Feature #12732].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62759 b2dd03c8-39d4-4d8f-98ff-823fe69b080e