Fix json marshal/unmarshal implementations

Fix marshal/unmarshal for:
- authentication_{cleartext_password, md5_password, ok, sasl, sasl_continue, sasl_final}
- error_response

authentication_ok.go

@@ -2,6 +2,7 @@ package pgproto3
 
 import (
 	"encoding/binary"
+	"encoding/json"
 	"errors"
 
 	"github.com/jackc/pgio"
@@ -37,3 +38,12 @@ func (src *AuthenticationOk) Encode(dst []byte) []byte {
 	dst = pgio.AppendUint32(dst, AuthTypeOk)
 	return dst
 }
+
+// MarshalJSON implements encoding/json.Marshaler.
+func (src AuthenticationOk) MarshalJSON() ([]byte, error) {
+	return json.Marshal(struct {
+		Type string
+	}{
+		Type: "AuthenticationOK",
+	})
+}