Fix equals comparsion returing False if both objects have nil Targets or Services.

[Dirbaio]

How to reproduce

• Deploy an nginx ingress controller
• Create an ingress with the Kubernetes service:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  annotations:
    ingress.kubernetes.io/secure-backends: "true"
    kubernetes.io/ingress.allow-http: "false"
    kubernetes.io/ingress.class: nginx
  name: kubernetes
  namespace: default
spec:
  rules:
  - host: example.com
    http:
      paths:
      - backend:
          serviceName: kubernetes
          servicePort: 443
        path: /
  tls:
  - hosts:
    - example.com
    secretName: tls-example-com

• Watch the ingress controller reload nginx over and over again, even if nothing changes

Why this happens

The Kubernetes service has nil in the address Target, unlike regular services. There's a bug in Endpoint.Equals() that returns false incorrectly if both endpoints have nil. This PR fixes it.

While looking at the other Equals functions, I noticed the same bug with Services in two places. I fixed them too, but I haven't checked if there is also a situation that can also cause endless reloads..

[k8s-reviewable]

This change is 

[coveralls]

Coverage increased (+0.02%) to 44.216% when pulling 94b28a9 on Dirbaio:master into 62aa9a4 on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@Dirbaio thanks!