Only pass first class to Pygments

This prevents an exception if something like the following is used:

~~~ {foo bar}
some code
~~~

lib/jekyll/converters/markdown.rb

@@ -15,7 +15,7 @@ def setup
 
             @renderer ||= Class.new(Redcarpet::Render::HTML) do
               def block_code(code, lang)
-                lang ||= 'text'
+                lang = lang && lang.split.first || "text"
                 output = add_code_tags(
                   Pygments.highlight(code, :lexer => lang, :options => { :encoding => 'utf-8' }),
                   lang