Using :into like this is the correct approach. The example in the docs doesn’t accumulate the body (it just writes it to the console). Frankly this is not a very helpful example and should be updated.
You absolutely should not rely on content-length. There have been serious security problems caused by people mistakenly trusting that header. The server can lie about it at will, maliciously. A timeout is also not a good approach as there is no guarantee you’re not downloading a large file very quickly!
