SFTP/Stream: use $scheme instead of 'sftp'
SFTP/Stream: use $scheme instead of 'sftp'

This makes it easier for someone to change the scheme (which can be
done by changng the stream_wrapper_register call at the bottom
of the file)

danog/phpseclib,splitice/phpseclib,creatuity/phpseclib,metaclassing/phpseclib,terrafrost/phpseclib,creatuity/phpseclib,melya/phpseclib,metaclassing/phpseclib,melya/phpseclib,splitice/phpseclib,thaigialai1987/phpseclib,paragonie-scott/phpseclib,danog/phpseclib,thaigialai1987/phpseclib,phpseclib/phpseclib,kenichiii/phpseclib,kenichiii/phpseclib,paragonie-scott/phpseclib