Simpler fix for credentials with characters such as '/'.

Thanks for describing the problem in great detail in PR #39 @opalmer.

I suspect there may be a much simpler fix, which I'd like to discuss. Consider this comment:

https://github.com/andygrunwald/go-gerrit/blob/70bbb05ea8c6502189a73862f956dac687b91075/gerrit.go#L101-L104

If I'm not mistaken, that URL gets parsed correctly by `url.Parse`. It's just that the URL itself is not correctly escaped, so it doesn't produce the results you want.

Let's use [`url.URL.String`](https://godoc.org/net/url#URL.String) method to construct a URL with "http" schema, "admin" username, "ZOSOKjgV/kgEkN0bzPJp+oGeJLqpXykqWFJpon/Ckg" password, and "localhost:38607" host:

```Go
u := &url.URL{
Scheme: "http",
User: url.UserPassword("admin", "ZOSOKjgV/kgEkN0bzPJp+oGeJLqpXykqWFJpon/Ckg"),
Host: "localhost:38607",
}
fmt.Println(u.String())

// Output: http://admin:ZOSOKjgV%2FkgEkN0bzPJp+oGeJLqpXykqWFJpon%2FCkg@localhost:38607
```

_(See on playground: https://play.golang.org/p/M3cq7xWI2eE.)_

Note that the `/` character in the password gets escaped to `%2F`.

When we parse that URL with `url.Parse`, it produces the expected results:

```Go
u, err := url.Parse("http://admin:ZOSOKjgV%2FkgEkN0bzPJp+oGeJLqpXykqWFJpon%2FCkg@localhost:38607")
if err != nil {
log.Fatalln(err)
}
fmt.Println(u.Scheme)
fmt.Println(u.User.Username())
fmt.Println(u.User.Password())
fmt.Println(u.Host)

// Output:
// http
// admin
// ZOSOKjgV/kgEkN0bzPJp+oGeJLqpXykqWFJpon/Ckg true
// localhost:38607
```

_(See on playground: https://play.golang.org/p/01GMpYMMzsw.)_

Notably, the original "ZOSOKjgV/kgEkN0bzPJp+oGeJLqpXykqWFJpon/Ckg" password is preserved.

So, I believe as long as the URL is correctly escaped, the logic added in #39 isn't needed and can be reverted. That would simplify the code. What do you think @opalmer?