Do not escape URI twice.

Drakma escapes URI twice when using `puri-unicode`.
```
(http-request "http://fr.wikipedia.org/wiki/Père")
```
results in
```
GET /wiki/P%25c3%25a8re HTTP/1.1
```
instead of
```
GET /wiki/P%c3%a8re HTTP/1.1
```

What happens with `puri-unicode` is that:
1. drakma calls make-instance 'puri:uri
2. initialize-instance computes uri-parsed-path because it was not provided to make-instance. It does this incorrectly, treating uri-path as being unescaped, thereas it either was already escaped, or is unaffected by escaping.
3. initialize-instance sets puri:uri-parsed-path to the computed (wrong) value
4. setting puri:uri-parsed-path recomputes and resets puri:uri-path to a wrong value

 Old `puri` is not affected just because it is does not compute uri-parsed-path preemptively (https://github.com/archimag/puri-unicode/blob/f5176bf4c8e05069d5ef1040df8489f686f73ac2/src.lisp#L252), and Drakma does not happen to trigger the code path that would corrupt URI. My patch is correct with both editions of `puri`.

I replaced `(or (puri:uri-path uri) "/")` with `(puri:uri-path uri)` because `puri:render-uri` does this substitution itself.