handle token headers without bearer string

[seboudry]

Hi!

When used with revomatico/kong-oidc without Authorization bearer token, request failed.

This MR change the behavior of this plugin to remove "bearer" string prefix in token header value only if exists.

Tested with this configuration of kong-oidc that used classic bearer token for non regression:

apiVersion: configuration.konghq.com/v1
kind: KongClusterPlugin
metadata:
  name: oidc-sso
  annotations:
    kubernetes.io/ingress.class: kong
plugin: oidc
config:
  client_id: xxxxxxxxxxxxxxxxxx
  client_secret: xxxxxxxxxxxxxxxxxxxxxxxxxx
  discovery: https://mydomain.com/auth/realms/master/.well-known/openid-configuration
  access_token_as_bearer: "yes"
  access_token_header_name: Authorization

Also tested without the two access_token_as_bearer and access_token_header_name that use X-Access-Token header without "bearer" string.

For reference, the configuration of jwt-keycloak plugin:

apiVersion: configuration.konghq.com/v1
kind: KongClusterPlugin
metadata:
  name: jwt-keycloak-sso
  annotations:
    kubernetes.io/ingress.class: kong
plugin: jwt-keycloak
config:
  cookie_names:
    - cookie_session
  allowed_iss:
    - https://mydomain.com/auth/realms/master
  header_names:
    - X-Access-Token
    - Authorization
  realm_roles:
    - a-role