Fix timing side-channel attack in hmac comparison

From the [crypto/hmac documentation](https://godoc.org/crypto/hmac):

>  Receivers should be careful to use Equal to compare MACs in order to avoid timing side-channels: 

```go
// CheckMAC returns true if messageMAC is a valid HMAC tag for message.
func CheckMAC(message, messageMAC, key []byte) bool {
	mac := hmac.New(sha256.New, key)
	mac.Write(message)
	expectedMAC := mac.Sum(nil)
	return hmac.Equal(messageMAC, expectedMAC)
}
```

In this library, `SigningMethodHMAC` currently uses `bytes.Equal` rather than the constant-time `hmac.Equal`.
Straightforward to fix :)