The HTTP proxy server's dispatch method decodes the Proxy-Authorization
Basic credential using base64.URLEncoding, but RFC 7617 specifies that
Basic authentication uses standard Base64 encoding (base64.StdEncoding).
The two encodings differ in their use of +/ vs -_ characters, so any
credential containing 0xff or other bytes that encode to / or + in
standard Base64 will fail to decode with URLEncoding, causing valid
authentication attempts to always be rejected with 407.
Additionally, the "Basic " scheme prefix check was case-sensitive, but
RFC 7235 section 2.1 specifies that auth-scheme is case-insensitive.
Fix both issues by switching to base64.StdEncoding and using
strings.ToLower for the scheme comparison.
Go's resp.Write() adds a "Content-Length: 0" header and it seems that
ffmpeg doesn't like this and immediately closes the proxy connection.
close: #1109
