internal/sessions: fix cookie clear session (#376)

CookieStore's ClearSession now properly clears the user session cookie by setting MaxAge to -1.

internal/sessions: move encoder interface to encoding package, and rename to MarshalUnmarshaler.
internal/encoding: move mock to own package
authenticate: use INFO log level for authZ error.

Signed-off-by: Bobby DeSimone <bobbydesimone@gmail.com>

internal/encoding/mock/mock_encoder_test.go

@@ -0,0 +1,26 @@
+package mock // import "github.com/pomerium/pomerium/internal/encoding/mock"
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestMockEncoder(t *testing.T) {
+	e := errors.New("err")
+	mc := Encoder{
+		MarshalResponse: []byte("MarshalResponse"),
+		MarshalError:    e,
+		UnmarshalError:  e,
+	}
+	s, err := mc.Marshal("test")
+	if err != e {
+		t.Error("unexpected Marshal error")
+	}
+	if string(s) != "MarshalResponse" {
+		t.Error("unexpected MarshalResponse error")
+	}
+	err = mc.Unmarshal([]byte("s"), "s")
+	if err != e {
+		t.Error("unexpected Unmarshal error")
+	}
+}