elementdecl.rb: suppress warnings

* lib/rexml/dtd/elementdecl.rb (PATTERN_RE): comment out overridden
  constant, and '_' is included in '\w'.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48541 b2dd03c8-39d4-4d8f-98ff-823fe69b080e