Http.requestURL throws exception on JS in 4.x

Test case:
```
class Test {
static function main() {
var url="https://worldtimeapi.org/api/timezone/America/Los_Angeles";

var result = haxe.Http.requestUrl( url );

trace(result);
}
}
```

try.haxe: https://try.haxe.org/#095cB22F

This code runs as expected in 3.4.7, but 4.x will result in `InvalidAccessError: Failed to set the 'responseType' property on 'XMLHttpRequest': The response type cannot be changed for synchronous requests made from a document.` (In JS).

Tested on Chrome and firefox.

As a general note: the async path works fine, so this isn't particularly high priority outside of niche cases where sync requests are preferable