Add failing tests for handling of charset and encoding
Fix filters for string bodies

The Plack::Response->_body method regenerates its value every time it's
called, so manipulating the array ref it returns doesn't change the response
that the client will see.
Fix the charset filter to actually work

If a charset filter was set, the code just died with an error like

  Can't locate object method "HASH(0x2f613f8)" via package ...
Remove extra newline