Support algorithm NONE when using the Authorization Code Flow

Currently, ID Token signed using the algorithm NONE(Plaintext JWT) is not supported in mod_auth_openidc.
The spec says it is possible to use "none" as the "alg"  when using the Authorization Code Flow.

Please refer to http://openid.net/specs/openid-connect-core-1_0.html#IDToken
```
ID Tokens MUST NOT use none as the alg value unless the Response Type used returns no ID Token
from the Authorization Endpoint (such as when using the Authorization Code Flow) and the Client
explicitly requested the use of none at Registration time.
```