Force users to validate the alg to protect against jwt vulnerability.

[nullboundary]

Published on https://github.com/dgrijalva/jwt-go README:

NOTICE: A vulnerability in JWT was recently published. As this library doesn't force users to validate the alg is what they expected, it's possible your usage is effected. There will be an update soon to remedy this, and it will likey require backwards-incompatible changes to the API. In the short term, please make sure your implementation verifies the alg is what you expect.

So in the mean time I implemented this fix following their instructions.

 // Don't forget to validate the alg is what you expect:
        if _, ok := token.Method.(*jwt.SigningMethodRSA); !ok {
            return nil, fmt.Errorf("Unexpected signing method: %v", token.Header["alg"])
        }

Also added the fixes from #34

[appleboy]

@nullboundary Please try https://github.com/appleboy/gin-jwt