Update replicationControllers get the namespace from the provided file "" does not match the namespace "default"

Steps to Reproduce:
1. create a replicationController

```kubectl create -f apache-slave-controller.json apacheSlaveController```

```
kubectl get replicationControllers 
NAME                    IMAGE(S)                         SELECTOR            REPLICAS
apacheSlaveController   10.66.10.152:5000/gouyang/http   name=apacheslave    4
```

2. update the json file, change the repilcas to 2
```
head -n 10 apache-slave-controller.json 
{
  "id": "apacheSlaveController",
  "kind": "ReplicationController",
  "apiVersion": "v1beta1",
  "desiredState": {
    "replicas": 2,
    "replicaSelector": {"name": "apacheslave"},
    "podTemplate": {
      "desiredState": {
         "manifest": {
```

3. update replicationControllers as below:
```
kubectl update -f apache-slave-controller.json 
F1125 14:30:35.855052   18936 update.go:51] the namespace from the provided file "" does not match the namespace "default". You must pass '--namespace=' to perform this operation.
```

Actual results:
it get error as step 3 show3

Expected results:
update successfully, the REPLICAS become to 2.

Fix:
Only do the namespace comparision check if it was actually populated in the supplied input file.  If omitted, we should not error and just pick up the current kubectl namespace.