make behavior of Fiber without block compatible with CRuby

mrbgems/mruby-fiber/src/fiber.c

@@ -73,7 +73,7 @@ fiber_init(mrb_state *mrb, mrb_value self)
   mrb_get_args(mrb, "&", &blk);
   
   if (mrb_nil_p(blk)) {
-    mrb_raise(mrb, E_FIBER_ERROR, "tried to create Fiber object without a block");
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "tried to create Fiber object without a block");
   }
   p = mrb_proc_ptr(blk);
   if (MRB_PROC_CFUNC_P(p)) {

mrbgems/mruby-fiber/test/fiber.rb

@@ -110,3 +110,7 @@ assert('Root fiber resume') do
   f.resume
   assert_false f.alive?
 end
+
+assert('Fiber without block') do
+  assert_raise(ArgumentError) { Fiber.new }
+end