Add 'iss' support for ruby-jwt

[ZhangHanDong]

add iss support, usage (in rspec):

example_payload = {"hello" => "world", "iss" => 'jwtiss'}
example_secret = 'secret'
example_jwt = 'eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJoZWxsbyI6IndvcmxkIiwiaXNzIjoiand0aXNzIn0.nTZkyYfpGUyKULaj45lXw_1gXXjHvGW4h5V7okHdUqQ'

JWT.decode(example_jwt, example_secret, true, {iss: 'jwtiss'})

or

begin
    JWT.decode(example_jwt, "secret"t, true, {iss: 'jwtiss'})
rescue JWT::InvalidIssuerError
    # Invalid issuer
end

[excpt]

Thanks @ZhangHanDong for the pull request.

All specs pass.

Merged! 🍻

[ZhangHanDong]

🍻

I looked forward to use 'iss' with new gem package . thanks @excpt

[excpt]

I'll write some documentation when I'm home and then I will push a new gem version. :)

[ZhangHanDong]

@excpt thanks。