Fixed a bug in AWS::EC2::Region where #client had a miss-configured ec2 endpoint.

EC2 regions provide access to resources in other regions, e.g.

  ec2.regions['us-west-2'].instances.map(&:id)

There was an issue where all EC2 service methods were correctly proxied
to the new region *except* for the #client.  Here is a demonstration of
that bug:

  ec2.regions['us-west-2'].client.config.ec2_endpoint
  #=> 'ec2.amazonaws.com'

This should have returned 'ec2.us-west-2.amazonaws.com'.  This patch
resolves this issue and updates a few incorrect tests.