Fix missing_default_stub_error message so array args are handled properly.

The source of this bug is that `raise_similar_message_args_error` receives
a list of args from multiple calls, e.g.:

foo.bar("a")
foo.bar(2, 3)

translates to `[["a"], [2, 3]]` being passed to it.  For the other `raise_...` methods, they receive an args list from a single call
(e.g. just `["a"]` for the first call in the example above).

`raise_similar_message_args_error` has slightly different logic to handle
the fact that it is different, but when `raise_missing_default_stub_error`
was added in 409955f5, it wrongly copied this logic.

Fixes #226.