Change default :next_token to nil when using stub_responses

[jkanywhere]

Please change the default value of :next_token to nil when using stub_responses to provide a better default experience by avoiding apparent duplication of responses when using the Resource interface.

When I stub a describe_instances response to contain 1 ec2 instance, for example, it is confusing to get back two from Aws::Resource::EC2#instances.

Recommended by @trevorrowe
See jkanywhere/aws-ruby-sample@189dadf and aws-samples/aws-ruby-sample#3 for context.

If you point me at the correct class or file that handles this feature I am happy to try my hand at resolving this.

[trevorrowe]

I've added this to my backlog and this will hopefully be part of the 2.1 release.