Require exact match when calling Get method within fake clientset

`Get` method within the fake clientset returns an object that would not be normally returned when using the real clientset. Reproducer:

```go
package main

import (
	v1 "k8s.io/api/core/v1"
	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/client-go/kubernetes/fake"
)

func main () {
	cm := &v1.ConfigMap{
		ObjectMeta: metav1.ObjectMeta{Namespace: metav1.NamespaceSystem, Name: "cm"},
		}

	client := fake.NewSimpleClientset(cm)
	obj, err := client.CoreV1().ConfigMaps("").Get("", metav1.GetOptions{})
	if err != nil {
		panic(err)
	}
	fmt.Printf("obj: %#v\n", obj)
}
```

stored under `test.go` of `github.com/kubernetes/kubernetes` (master HEAD) root directory and ran:

```sh
$ go run test.go
obj: &v1.ConfigMap{TypeMeta:v1.TypeMeta{Kind:"", APIVersion:""}, ObjectMeta:v1.ObjectMeta{Name:"cm", GenerateName:"", Namespace:"kube-system", SelfLink:"", UID:"", ResourceVersion:"", Generation:0, CreationTimestamp:v1.Time{Time:time.Time{wall:0x0, ext:0, loc:(*time.Location)(nil)}}, DeletionTimestamp:(*v1.Time)(nil), DeletionGracePeriodSeconds:(*int64)(nil), Labels:map[string]string(nil), Annotations:map[string]string(nil), OwnerReferences:[]v1.OwnerReference(nil), Finalizers:[]string(nil), ClusterName:"", ManagedFields:[]v1.ManagedFieldsEntry(nil)}, Data:map[string]string(nil), BinaryData:map[string][]uint8(nil)}
```

As you can see fake clientset with a "test" configmap is created. When getting the object through the clientset back, I intentionally set the object name to an empty string. I would expect to get an error saying config map "" was not found. However, I get "test" configmap instead.

Reason for that is inside implementation of `filterByNamespaceAndName` private function:
```go
func filterByNamespaceAndName(objs []runtime.Object, ns, name string) ([]runtime.Object, error) {
	var res []runtime.Object

	for _, obj := range objs {
		acc, err := meta.Accessor(obj)
		if err != nil {
			return nil, err
		}
		if ns != "" && acc.GetNamespace() != ns {
			continue
		}
		if name != "" && acc.GetName() != name {
			continue
		}
		res = append(res, obj)
	}

	return res, nil
}
```

When `name` is empty, `name != "" && acc.GetName() != name` condition is false and thus `obj` is consider as a fit.

[1] https://github.com/kubernetes/client-go/blob/master/testing/fixture.go#L481-L493

Kubernetes-commit: d32c76fc03381784516c47cb1bf62ef932189afa

pkg/describe/versioned/describe_test.go

@@ -3133,7 +3133,7 @@ Spec:
 		},
 	})
 	d := NetworkPolicyDescriber{versionedFake}
-	out, err := d.Describe("", "network-policy-1", describe.DescriberSettings{})
+	out, err := d.Describe("default", "network-policy-1", describe.DescriberSettings{})
 	if err != nil {
 		t.Errorf("unexpected error: %s", err)
 	}