Fix MRB_WORD_BOXING by correctly telling GC which mrb_value types are immediate.

include/mruby/value.h

@@ -398,7 +398,7 @@ struct RObject {
 #define mrb_obj_ptr(v)   ((struct RObject*)(mrb_ptr(v)))
 /* obsolete macro mrb_object; will be removed soon */
 #define mrb_object(o) mrb_obj_ptr(o)
-#define mrb_immediate_p(x) (mrb_type(x) <= MRB_TT_CPTR)
+#define mrb_immediate_p(x) (mrb_type(x) < MRB_TT_HAS_BASIC)
 #define mrb_special_const_p(x) mrb_immediate_p(x)
 
 struct RFiber {