Credentials::decode() should return Result instead of Option

[bwalter]

I'm using "headers" together with warp to decode basic auth headers. With the current behaviour, my web server would panic if the header value does not start with "Basic ".

It would be great in my opinion if Credentials::decode() could return a Result instead of an Option. In the above use case, Basic::decode() would return an Err instead of panicking.

Current behavior:

let header_value = http::header::HeaderValue::from_static("Bazic xxx");
let basic = Basic::decode(&header_value);  // panic

Feature request (with error handling):

let header_value = http::header::HeaderValue::from_static("Bazic xxx");
let basic = Basic::decode(&header_value).map_err(...)?;  // proper error handling