Username/password encoding in HTTP basic auth is currently broken for
non-ASCII password.

Example with user `user` and password `unusual-char-é`. With curl it
works as expected:

```shell
curl -v http://user:unusual-char-%C3%A9@localhost:5984/
```

```
> GET / HTTP/1.1
> Authorization: Basic YWxpY2U6YWRyaWVuPTohw6k=
>
< HTTP/1.1 200 OK
```

But with couchdb-python the string is decoded from `utf-8` then
re-encoded into `latin1`, causing an incorrect Authorization header:

```python
url = 'http://user:unusual-char-%C3%A9@localhost:5984/'
couchdb.Server(url).version()
```

```
> GET / HTTP/1.1
> Authorization: Basic dXNlcjp1bnVzdWFsLWNoYXIt6Q==
>
< HTTP/1.1 401 Unauthorized
```

This patch fixes this wrong encoding charset by using `utf-8` (used by
default by CouchDB) instead of `latin1`.

Closes: djc#301