rb_str_modify_expand: clear the string coderange

[Bug #19468]

b0b9f7201a errornously stopped
clearing the coderange.

Since `rb_str_modify` clears it, `rb_str_modify_expand`
should too.

string.c

@@ -2473,6 +2473,7 @@ rb_str_modify_expand(VALUE str, long expand)
     else if (expand > 0) {
         RESIZE_CAPA_TERM(str, len + expand, termlen);
     }
+    ENC_CODERANGE_CLEAR(str);
 }
 
 /* As rb_str_modify(), but don't clear coderange */