Resolves cases where backtrace is a String

Such as https://github.com/celluloid/celluloid/blob/fixed-stackdump/lib/celluloid/tasks/task_fiber.rb#L40

Without this, the above code ( and other code returning a `String` for a backtrace ) produces this:

```
NoMethodError undefined method `each' for #<String:0x3f250acc>
```
/cc: @tarcieri @halorgium -- if backtraces are always supposed to ALWAYS be of kind `Array` -- then `TaskFiber.backtrace` ought to be changed instead, and I will do that. Not sure if it's better to catch-all this way, or force compliance.