Force encoding to UTF8 before passing content to highlighter

Rouge expects content to be a valid UTF8 string.  If the string does not contain UTF-8 Characters, it is casted to ASCII-8BIT, Triggerring an error:


To reproduce:
**index.slim**
```
- code('html') do
  == "BOB".force_encoding(Encoding::UTF_8)
```
=>
```
Bad encoding: ASCII-8BIT,BINARY. Please convert your string to UTF-8.
```

**index.slim**
```
- code('html') do
  == "BOBé".force_encoding(Encoding::UTF_8)
```
=> Works.


We make sure "content" passed to highlighter a valid UTF8 string.