Utils: deep_merge_hashes should also merge default_proc

If the target hash's default_proc isn't set, overwrite with the new hash's default_proc.

/cc #4064

lib/jekyll/utils.rb

@@ -34,6 +34,10 @@ module Jekyll
         target[key] = overwrite[key]
       end
 
+      if target.default_proc.nil?
+        target.default_proc = overwrite.default_proc
+      end
+
       target
     end