Don't silently ignore expressions

Currently if you do something like

``` ruby
assert_difference "Exp1", "Exp2" do
  foo
end
```

Exp2 is ignored because it's taken as the error message. Instead one should write

``` ruby
assert_difference ["Exp1", "Exp2"] do
  foo
end
```

but there's no error pushing one to write it the correct way. Fix the API so this thing cannot happen.
