Use URI::RFC2396_Parser#regex explicitly.

In background, URI will change DEFAULT_PARSER to RFC2396_Parser.

ruby/uri#107

After that, we lost some of regex pattern like UNSAFE.

mustermann/lib/mustermann/ast/translator.rb

@@ -118,7 +118,7 @@ def decorator_for(node)
 
       # @return [String] escaped character
       # @!visibility private
-      def escape(char, parser: URI::DEFAULT_PARSER, escape: parser.regexp[:UNSAFE], also_escape: nil)
+      def escape(char, parser: URI::DEFAULT_PARSER, escape: URI::RFC2396_Parser.new.regexp[:UNSAFE], also_escape: nil)
         escape = Regexp.union(also_escape, escape) if also_escape
         char.to_s =~ escape ? parser.escape(char, Regexp.union(*escape)) : char
       end