Use rb_inspect for Ractor error

Previously the object was used directly, which calls `to_s` if defined.
We should use rb_inspect to get a value suitable for display to the
programmer.

Author: jhawthorn

ractor.c

@@ -3163,7 +3163,7 @@ make_shareable_check_shareable(VALUE obj)
             return traverse_cont;
         }
         else {
-            rb_raise(rb_eRactorError, "can not make shareable object for %"PRIsVALUE, obj);
+            rb_raise(rb_eRactorError, "can not make shareable object for %+"PRIsVALUE, obj);
         }
     }
 

test/ruby/test_ractor.rb

@@ -60,6 +60,14 @@ def test_shareability_of_method_proc
     assert_unshareable(x, "can not make shareable object for #<Method: String(Kernel)#itself()>", exception: Ractor::Error)
   end
 
+  def test_shareability_error_uses_inspect
+    x = (+"").instance_exec { method(:to_s) }
+    def x.to_s
+      raise "this should not be called"
+    end
+    assert_unshareable(x, "can not make shareable object for #<Method: String#to_s()>", exception: Ractor::Error)
+  end
+
   def test_default_thread_group
     assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
     begin;