NoMethodError: undefined method `zero?' for nil:NilClass if JWT has no 'alg' field

[bouk]

For example:

 JWT.decode('e30K.e30K.e30K')
=> NoMethodError: undefined method `zero?' for nil:NilClass

I'd expect this to throw a JWT::IncorrectAlgorithm, but casecmp here:

ruby-jwt/lib/jwt/decode.rb

Line 46 in 2cea14f

allowed_algorithms.any? { |alg| alg.casecmp(header['alg']).zero? }

returns nil if alg is not set, so we get an unexpected NoMethodError.