Why does mini-unit change the exception in the test below?

--start code----------------------------------------
require 'test/unit'

def epic_fail
   fail RuntimeError.new("Oops")
end

class FailureTest < Test::Unit::TestCase
   def test_failure
     epic_fail
   rescue Exception => ex
     puts "Expecting RuntimeError, got: #{ex.class}"
   end
end
--end code----------------------------------------

Here's the output:

    $ ruby19 test_fail.rb
    Loaded suite test_fail
    Started
    Expecting RuntimeError, got: Mini::Assertion
    .
    Finished in 0.000435 seconds.

    1 tests, 1 assertions, 0 failures, 0 errors

-- 
-- Jim Weirich
-- jim.weirich / gmail.com