webrick: WEBrick::Log requires path arg when given string

Allowing a user to specify "| command" via Kernel#open is
nonsensical since we never read from the resultant IO.

* lib/webrick/log.rb (initialize): replace Kernel#open with File.open
  [Misc #14216]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61400 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/webrick/log.rb

@@ -51,7 +51,7 @@ module WEBrick
       @level = level || INFO
       case log_file
       when String
-        @log = open(log_file, "a+")
+        @log = File.open(log_file, "a+")
         @log.sync = true
         @opened = true
       when NilClass