go.crypto/ssh: fix certificate parsing/marshaling.

The change to add the PublicKey interface accidentally caused certificate handling to expect an extra copy of the private key algorithm name in the binary representation. This change adapts a suitable parsing API and adds a test to ensure that cert handling isn't easily broken in the future.

R=agl, hanwen, jmpittman
CC=golang-dev
https://golang.org/cl/13272055

ssh/agent.go

@@ -100,7 +100,7 @@ func (ak *AgentKey) String() string {
 
 // Key returns an agent's public key as one of the supported key or certificate types.
 func (ak *AgentKey) Key() (PublicKey, error) {
-	if key, _, ok := parsePubKey(ak.blob); ok {
+	if key, _, ok := ParsePublicKey(ak.blob); ok {
 		return key, nil
 	}
 	return nil, errors.New("ssh: failed to parse key blob")