improved reporting of failed expression in assert_difference

Patch improves message output when an expression specified in #assert_difference fails. This is useful for when there are multiple expressions provided as an array to help identify which one was actually in error.

Currently assert_difference just outputs that the expected and actual value was, but not what actually failed.

assert_difference ['Post.count', 'current_user.posts.count'] do
  save_a_new_post
end

If one of the expressions fails in the above code, there is no way to tell from the assertion failure message which one: <1> expected but was <0>.

With the patch, the message will be: <current_user.posts.count> was expression that failed. <1> expected but was <0>.