fix: force StringIO encoding to ASCII_8BIT

`render` method in `Prawn::Document` uses a `StringIO` instance as default output.
But on JRuby for Windows this class uses a default encoding of `UTF-8` instead of `ASCII_8BIT`, than breaking in` render_header` method by pushing non-UTF characters in the `StringIO` instance.

It was also breaking a lot of tests (I'm not listing all of them. Should be at least every in-memory `render` calls).

Environment:
* Windows 7 x64 SP1
* Java 1.7rev51
* JRuby 1.7.10