Sticky backend isn't detected as being unavailable (with consulCatalog)

[aksdb]

Do you want to request a feature or report a bug?

BUG (probably a regression of #1299)

What did you do?

Using the consulCatalog I started a service redundant with the tag traefik.backend.loadbalancer.sticky=true. I started a request against that endpoint using a browser then took down the service instance specified in the cookie.

What did you expect to see?

A response from the other, still intact instance as well as a new stick cookie referring to the new service.

What did you see instead?

"Bad Gateway"
Reloading multiple times didn't work. Waiting several minutes didn't work. Only removing the cookie worked.

Output of traefik version:

Version:      v1.4.0-rc3
Codename:     roquefort
Go version:   go1.9
Built:        2017-09-18_04:38:27PM
OS/Arch:      linux/amd64

[ldez]

Fixed by #2216