Site#configure_theme: do not set theme unless it's a string

Some previous ad-hoc 'themes' used this configuration option to store a hash of values.
In that case, we should simply pretend we have no theme.

lib/jekyll/site.rb

@@ -424,7 +424,7 @@ module Jekyll
     private
     def configure_theme
       self.theme = nil
-      self.theme = Jekyll::Theme.new(config["theme"]) if config["theme"]
+      self.theme = Jekyll::Theme.new(config["theme"]) if config["theme"].is_a?(String)
     end
 
     private