`scheme://user:@...', `scheme://:password@...' and
  `scheme://:@...' are parsed, an empty user name or password
  should be allowed and represented as it is.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8@11751 b2dd03c8-39d4-4d8f-98ff-823fe69b080e