Add specs for recursion guard on `<=>`

This adds specs to Time and String to ensure that the reverse comparison
they perform as part of the `<=>` method does not result in a stack
overflow. Related to [Ruby Bug
test suite (for the specific case of comparing a Time object to a String
object)
[here](https://github.com/ruby/ruby/blob/trunk/test/ruby/test_comparable.rb#L73)
Update wording for spec
Remove unnecessary "should not raise" check
Merge pull request #263 from jballanc/invcmp-spec

Add specs for recursion guard on `<=>`