No error when assigning string to string element via set-path

I've noticed, that even in early R3 version there is:

```rebol
s: "aaa" s/2: "XX"
;== "XX" ;<-- string is returned
s
;== "aXa" ;<-- only the char is modified
```

while in Red and Rebol2 it is:

```red
s: "aaa" s/2: "XX"
;*** Script Error: invalid argument: "XX"
```
