escape.c: check argument
* ext/cgi/escape/escape.c (optimized_unescape): check
accept_charset encoding argument.
share `@@accept_charset`
* lib/cgi/{core,util}.rb: include CGI::Util not only extending, to
share `@@accept_charset` class variable, so that it is always
accessible. [ruby-core:80986] [Bug #13539]
merge revision(s) 57501,58576: [Backport #13539]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@59511 b2dd03c8-39d4-4d8f-98ff-823fe69b080e