Empty AuthScheme not possible for headers

[418Coffee]

Hello,

I'm trying to do something similar to #103, extract a JWT from a custom header field.

{
    "X-Auth": "eyJ....."
}

I believe currently, it's impossible to achieve this, because an empty AuthScheme value gets interpreted as default and set to "Bearer":

if cfg.AuthScheme == "" {
    cfg.AuthScheme = "Bearer"
}

Perhaps we can default AuthScheme to "Bearer" if AuthScheme == "" && TokenLookup == defaultTokenLookup.
Please let me know what you think about this. I would be happy to make a PR.

[gaby]

@418Coffee Send a PR, as long as it's backwards compatible should be fine. :-)