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.

error.c

@@ -72,6 +72,7 @@ static VALUE rb_mWarning;
 static VALUE rb_cWarningBuffer;
 
 static ID id_warn;
+static ID id_category;
 
 extern const char ruby_description[];
 
@@ -231,11 +232,11 @@ rb_warning_s_aset(VALUE mod, VALUE category, VALUE flag)
 
 /*
  * call-seq:
- *    warn(msg, **kw)  -> nil
+ *    warn(msg, category: nil)  -> nil
  *
  * Writes warning message +msg+ to $stderr. This method is called by
  * Ruby for all emitted warnings. A +category+ may be included with
- * the warning.
+ * the warning, but is ignored by default.
  */
 
 static VALUE
@@ -243,8 +244,11 @@ rb_warning_s_warn(int argc, VALUE *argv, VALUE mod)
 {
     VALUE str;
     VALUE opt;
+    VALUE category;
 
     rb_scan_args(argc, argv, "1:", &str, &opt);
+    if (!NIL_P(opt)) rb_get_kwargs(opt, &id_category, 0, 1, &category);
+
     Check_Type(str, T_STRING);
     rb_must_asciicompat(str);
     rb_write_error_str(str);
@@ -2749,6 +2753,7 @@ Init_Exception(void)
     id_errno = rb_intern_const("errno");
     id_i_path = rb_intern_const("@path");
     id_warn = rb_intern_const("warn");
+    id_category = rb_intern_const("category");
     id_top = rb_intern_const("top");
     id_bottom = rb_intern_const("bottom");
     id_iseq = rb_make_internal_id();

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|