Preserve hashes in url() function

`url()` function should preserve hashes after base64 conversion.

For example

```css
div {
  background: url("circle.svg#some-id");
}
```

should compiles to

```css
div {
  background: url("data:image/svg+xml;base64,PD94b <…> z4K#some-id");
}
```

but now it compiles to

```css
div {
  background: url("data:image/svg+xml;base64,PD94b <…> z4K");
}
```