use `request.header(AUTH_HEADER)` instead of `request.headers[AUTH_HEADER]`

[kasir-barati]

Hi dear reader,

I think it is best to use req.header('header-case-insensitive-name') here. TBH IDK much about how express, or fastify works -- I mean normalizing headers. But what I am sure about it that it makes this lib a bit more resilient.

READ more.

I was writing some unit test and I found out how passport-jwt is working.