Avoid infinite loop when converting objects to strings.

src/object.c

@@ -338,6 +338,7 @@ mrb_convert_type(mrb_state *mrb, mrb_value val, enum mrb_vtype type, const char
   if (mrb_type(val) == type) return val;
   v = convert_type(mrb, val, tname, method, TRUE);
   if (mrb_type(v) != type) {
+    if (type == MRB_TT_STRING) return mrb_any_to_s(mrb, val);
     mrb_raisef(mrb, E_TYPE_ERROR, "%v cannot be converted to %s by #%s", val, tname, method);
   }
   return v;

src/string.c

@@ -1121,6 +1121,7 @@ mrb_str_to_str(mrb_state *mrb, mrb_value str)
     return mrb_sym_str(mrb, mrb_symbol(str));
   case MRB_TT_FIXNUM:
     return mrb_fixnum_to_str(mrb, str, 10);
+  case MRB_TT_SCLASS:
   case MRB_TT_CLASS:
   case MRB_TT_MODULE:
     return mrb_mod_to_s(mrb, str);