Encode strings as UTF-8 when it has wide characters

This has been controversial for so long, and my canned response is that an app returning data that breaks PSGI specification is ok to break, and should these kind of errors be handled by Lint in the development.

Meanwhile, a) realistically it's not cool for app mistakes to be able to crash servers and b) if some of these PSGI violation only happens *sometimes* in the runtime, catching them with Lint might be difficult as well.

There should always be a line drawn, since there are many other ways to break HTTP::Server::PSGI as well by returning (for example) a hash for headers instead of arrays, and handling all of these mistakes in the server isn't realistic (and could make the server slow).

This should probably be documented as part of the spec or guideline, but here's a quick fix on the reference server to catch such errors.

```
> perl -S plackup  -E dev -e ' sub{[200,["Content-Type","text/plain"],["\x{3092}"]]}'
Wide character in syswrite at /Users/miyagawa/.plenv/versions/5.18.1/lib/perl5/5.18.1/darwin-2level/IO/Handle.pm line 481.
```