Return SetStackPolicyResponse - Fixes #2346 #2639

Just return the ‘SetStackPolicyResponse’ whether its
‘SetStackPolicyResult’ or 'ResponseMetadata’
Fix set_stack_policy test
Remove unnecessary assertion
Merge pull request #2822 from micahhausler/bugfix/cloudformation_set_stack_policy

Return SetStackPolicyResponse - Fixes #2822, #2346 #2639.