Setting scheme does not work as expected

Is this a bug?

Getting scheme works:

```
(-> "https://foo.bar" fish/uri fish/scheme str)
"https"
```

Changing scheme works:
```
user> (-> "https://foo.bar" fish/uri (fish/scheme "http") str)
"http://foo.bar"
```

Setting scheme when scheme was previously nil does not:
```
(-> "foo.bar" fish/uri (fish/scheme "http") str)
"http:foo.bar"
```