Runtime context is not reset before released to the pool

[lovromazgon]

When RuntimeContext is released back into the pool (here), it is not reset, which can cause the context passed to json.MarshalContext to be leaked into a different call to json.Marshal.

Failing test:

type Foo struct{}

func (Foo) MarshalJSON(ctx context.Context) ([]byte, error) {
	if ctx != nil {
		if val := ctx.Value("ctxKey"); val != nil {
			return []byte(`"` + val.(string) + `"`), nil
		}
	}
	return []byte(`"foo"`), nil
}

func TestContext(t *testing.T) {
	foo := Foo{}

	// first marshal with context
	ctx := context.WithValue(context.Background(), "ctxKey", "bar")
	out, _ := json.MarshalContext(ctx, foo)
	if string(out) != `"bar"` {
		t.Fatalf(`expected "bar", got %v`, string(out))
	}

	// second marshal without context (bug: it actually uses the previous context)
	out, _ = json.Marshal(foo)
	if string(out) != `"foo"` {
		t.Fatalf(`expected "foo", got %v`, string(out))
	}
}

Results in:

    context_test.go:34: expected "foo", got "bar"