Default to RFC 7638 kid fingerprint generation

The switch from the `json-jwt` to `jwt` gem in #177 changed the
default `kid` generation from RFC 7638
(https://www.rfc-editor.org/rfc/rfc7638) to a format based on the
SHA256 digest of the key elements.

However, clients may fail if the the `kid` generated by `IdToken` does
not match a key listed in JWKS discovery endpoint, which may be
implemented by the application using RFC 7638-based `kid` values. To
restore the previous behavior, applications have to set a global
setting:

```
JWT.configuration.jwk.kid_generator_type = :rfc7638_thumbprint
```

However, relying on this global setting is not ideal since other keys
may depend on the legacy `kid` values.

In keeping with semantic versioning, restore the `kid` generation to
RFC 7638. Whether this should be customizable can be discussed later.

Closes #193

lib/doorkeeper/openid_connect.rb

@@ -48,7 +48,7 @@ def self.signing_key
         else
           OpenSSL::PKey.read(configuration.signing_key)
         end
-      ::JWT::JWK.new(key)
+      ::JWT::JWK.new(key, { kid_generator: JWT::JWK::Thumbprint })
     end
 
     def self.signing_key_normalized

spec/rails_helper.rb

@@ -53,7 +53,6 @@
 
   # Reinitialize configuration after each example
   config.after do
-    load Rails.root.join('config/initializers/jwt.rb')
     load Rails.root.join('config/initializers/doorkeeper.rb')
     load Rails.root.join('config/initializers/doorkeeper_openid_connect.rb')
   end