[2.0] Fixes streamed responses when using Octane

[joedixon]

When using Octane of Vapor and returning a StreamedResponse such as Storage::download('my-file.png');, the file is downloaded with no content.

The following code is the issue:

$content = $response instanceof BinaryFileResponse
            ? $response->getFile()->getContent()
            : $response->getContent();

Calling getContent() on a StreamedResponse returns false and so we end up with a file with no content.

With Lambda, we can't stream directly to the client. Instead, this PR captures the content from the stream and returns it.