Crypting algorithm H instead of R256

The algorithm used in fireauth is HS256 as specified in [generator.go](https://github.com/zabawaba99/fireauth/blob/master/generator.go)

```go
// Firebase specific values for header  
const (  
	TokenAlgorithm = "HS256"  
	TokenType      = "JWT"  
)  
```

Firebase seems to use RS256 as referenced here:
https://firebase.google.com/docs/auth/admin/verify-id-tokens