uri/common.rb: use negative look-ahead

* lib/uri/common.rb (URI.decode_www_form_component): use negative
  look-ahead instead of nested repeat operators, to get rid of
  backtrack explosion.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47273 b2dd03c8-39d4-4d8f-98ff-823fe69b080e