url safe encode and decode

[beekus]

This issue pops up when I am trying to make a JWT assertion to a service that isn't implemented using this gem. I think there may be an issue with base64url_encode.

body = JWT.encode({'foo' => 'bar'}, nil, nil).split('.').last
=> "eyJmb28iOiJiYXIifQ"
Base64.decode64(body)
=> "{\"foo\":\"bar\""

Shouldn't there be a closing curly brace on the above? I think you can use Base64.urlsafe_encode64 and Base64::urlsafe_decode64 instead of what you are doing in lines 38-45. I can submit a pull request with this unless there are any objections.

[beekus]

further reading leads me to believe that i am wrong.