`ruby-rewrite` fails to actually rewrite the input file when a rewriter is loaded.  This fixes the issue.

At the end of the `@rewriters.each` loop, the value of `buffer.name` is no longer valid due to debugging annotations:

```ruby
        new_buffer = Source::Buffer.new(initial_buffer.name +
                                    '|after ' + rewriter_class.name)
```

This preserves the original name, allowing the file to be properly rewritten.
Please use `Dir.mktmpdir` for this.