Avoid array creation when checking if source is UTF-8 (#1141)

This commit replaces a check that involved the creation of an arrays of
strings with a direct comparison against those three strings.

lib/rouge/lexer.rb

@@ -263,7 +263,9 @@ def mimetypes(*mts)
 
       # @private
       def assert_utf8!(str)
-        return if %w(US-ASCII UTF-8 ASCII-8BIT).include? str.encoding.name
+        encoding = str.encoding.name
+        return if encoding == 'US-ASCII' || encoding == 'UTF-8' || encoding == 'ASCII-8BIT'
+
         raise EncodingError.new(
           "Bad encoding: #{str.encoding.names.join(',')}. " +
           "Please convert your string to UTF-8."