[AWS|EC2] request_spot_instances.rb requires that date parameters be iso8601
Fix subnet mocking, related to #2510
Merge pull request #2518 from drcapulet/master

Fix subnet mocking, related to #2510
Merge pull request #2514 from fcheung/request_spot_instances_time_coercion

[AWS|EC2] request_spot_instances.rb requires that date parameters be iso...