Fix: do not try to parse a rsa key if algo is hmac

[malud]

No description provided.

[filex]

review ok

tested with HS256 and plain (i.e. binary) and base64 encoded keys from jwt.io.

[filex]

example config:

definitions {
  jwt "JWTToken" {
    header = "Authorization"
    signature_algorithm = "HS256"
    key = "asdf"
  }
}

before:

panic: loading jwt "JWTToken" definition failed: asn1: syntax error: truncated tag or length

(or different messages depending on length of the key).

after: couper starts and can validate tokens from jwt.io with that key.