Authorization scheme for HTTP Basic Authentication should be case insensitive

Hi,

According to [RFC 2617](https://tools.ietf.org/html/rfc2617#section-1.2), the authorization scheme should be case insensitive; thus, `Basic` and `basic` should both be valid.

Here's the relevant part:

```
HTTP provides a simple challenge-response authentication mechanism
that MAY be used by a server to challenge a client request and by a
client to provide authentication information. It uses an extensible,
case-insensitive token to identify the authentication scheme...
```

Thanks!

No source file, test, or entry point is named in the issue. Locate the HTTP Basic Authentication authorization-scheme parsing in the repository, then add coverage showing that both `Basic` and `basic` are accepted, using RFC 2617 as the behavior reference.

Written by the indexing model from the issue text.