make time_for always raise an ArgumentError if something goes wrong

lib/sinatra/base.rb

@@ -421,7 +421,7 @@ module Sinatra
         Time.parse value.to_s
       end
     rescue ArgumentError => boom
-      raise boom.to_s
+      raise boom
     rescue Exception
       raise ArgumentError, "unable to convert #{value.inspect} to a Time object"
     end

test/helpers_test.rb

@@ -679,7 +679,7 @@ class HelpersTest < Test::Unit::TestCase
     end
 
     it 'fails when Time.parse raises an ArgumentError' do
-      assert_raise(RuntimeError) { get '/boom' }
+      assert_raise(ArgumentError) { get '/boom' }
     end
   end