Requests not being signed correctly

[fiddlerwoaroof]

This call fails with an error saying that the signature provided in the Authorization header didn't match the signature Amazon computed:

(aws-sdk/services/cloudformation:list-stacks 
 :stack-status-filter '("UPDATE_COMPLETE" "UPDATE_IN_PROGRESS"
                        "UPDATE_COMPLETE_CLEANUP_IN_PROGRESS" "UPDATE_ROLLBACK_COMPLETE"
                        "UPDATE_ROLLBACK_IN_PROGRESS" "UPDATE_ROLLBACK_FAILED" 
                        "UPDATE_ROLLBACK_COMPLETE_CLEANUP_IN_PROGRESS" "CREATE_FAILED"
                        "CREATE_COMPLETE" "CREATE_IN_PROGRESS"))

I think this PR fixes it upstream: rotatef/aws-sign4#6, I'm just reporting this here in case that PR doesn't get merged.