Make Time#to_r consistent with MRI

Hello,

There is an inconsistency between MRI and JRuby when dealing with a `Time` object then calling `to_r` on it. JRuby returns simply the numerator of the `Rational` if the latter has 1 as its denominator.

The commit introducing this feature is jruby/jruby@e748f0d6365615ecbf031fe59ed2a99d098aa6f5 ; there is no information about the addition of this `if` call on the [original ticket](http://jira.codehaus.org/browse/JRUBY-4462) though so we can simply remove it.

This makes the [following assertion](https://github.com/jruby/jruby/blob/cd3390067c9184e9239cd245c4f1f07b9f775d51/spec/ruby/core/time/to_r_spec.rb#L9) to pass now.

Have a nice day.