Make Warning.warn accept only category keyword

In general accepting arbitrary keywords is a bad idea unless you are
delegating keywords or acting on arbitrary keywords.  In this case,
the category keyword is ignored, and it's less error prone to not
ignore all keywords.

test/ruby/test_exception.rb

@@ -925,8 +925,8 @@ $stderr = $stdout; raise "\x82\xa0"') do |outs, errs, status|
       remove_method :warn
 
       if category
-        define_method(:warn) do |str, **kw|
-          warning << [str, kw[:category]]
+        define_method(:warn) do |str, category: nil|
+          warning << [str, category]
         end
       else
         define_method(:warn) do |str|