Fix garbled characters with code blocks including utf-8 strings

The utf-8 strings in code blocks seem to be garbled.

So I add a spec with a utf-8 fixture and it is failed:

```
Failures:

  1) Gimli::Markup::Renderer should
     Failure/Error: markup.render.should == output
       expected: "<div class=\"CodeRay\">\n  <div class=\"code\"><pre>まつもとゆきひろ</pre></div>\n</div>\n\n"
            got: "<div class=\"CodeRay\">\n  <div class=\"code\"><pre>������������������������</pre></div>\n</div>\n\n" (using ==)
       Diff:
       @@ -1,4 +1,4 @@
        <div class="CodeRay">
       -  <div class="code"><pre>まつもとゆきひろ</pre></div>
       +  <div class="code"><pre>������������������������</pre></div>
        </div>
     # ./spec/gimli/markup/renderer_spec.rb:29:in `block (2 levels) in <top (required)>' 
```

The changes fix this issue.