UTF-8 strings stored as binary, load as ASCII-8BIT

Some valid UTF-8 strings lose their encoding when passed through YAML:

``` ruby
2.2.1 :001 > "\xEF\xBF\xBD\x1F".encoding
 => #<Encoding:UTF-8> 
2.2.1 :002 > "\xEF\xBF\xBD\x1F".valid_encoding?
 => true 
2.2.1 :003 > "\xEF\xBF\xBD\x1F".to_yaml
 => "--- !binary |\n77+9Hw==\n\n" 
2.2.1 :004 > YAML.load("\xEF\xBF\xBD\x1F".to_yaml).encoding
 => #<Encoding:ASCII-8BIT> 
```
