assertions.rb: fix omitted message

* lib/test/unit/assertions.rb (assert_raise_with_message): remove
  omitted message by splatting nil.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43374 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-10-21

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43375 b2dd03c8-39d4-4d8f-98ff-823fe69b080e