Do not fail on undefined request scheme

[OndraM]

When using local PHP server, the 'REQUEST_SCHEME' is empty for me, causing the script to fail:

Warning: Undefined array key "REQUEST_SCHEME"

This will fallback to 'http' when REQUEST_SCHEME is set.

[roelofjan-elsinga]

This referrer currently breaks my installation as I'm only using this package with a local CLI script. This PR would fix my problem.

[metzg]

In my case (CLI script), the following keys were missing: REQUEST_SCHEME, HTTP_HOST, REQUEST_URI. Would you consider adding those two other keys in your patch?

A workaround I found without this patch was to set those values manually before calling getImage, but it's not ideal:

$_SERVER['REQUEST_SCHEME'] = $_SERVER['REQUEST_SCHEME'] ?? 'https';
$_SERVER['HTTP_HOST'] = $_SERVER['HTTP_HOST'] ?? 'example.com';
$_SERVER['REQUEST_URI'] = $_SERVER['REQUEST_URI'] ??  '';

// ...

$foo->getImage();

Thanks,

[DantSu]

Yes, this workaround is a better way than the patch, because you can set custom value.