Knative implementation returns init response on non-stem cell first runs

On the first run of non-stem cell (environmentally initialized containers), return the initialization response rather than the return of the run.

Replication steps:
service.yaml:
```yaml
apiVersion: serving.knative.dev/v1
kind: Service
metadata:
name: greetings-builtin-code
namespace: default
spec:
template:
spec:
containers:
- image: openwhisk/action-nodejs-v12
env:
- name: __OW_RUNTIME_PLATFORM
value: knative
- name: __OW_ACTION_NAME
value: greetings-builtin-code
- name: __OW_ACTION_CODE
value: "function main(params) {return {payload: 'Hello ' + params.name + ' from ' + params.place + '!'};}"
```
After deployment,
```bash
kubectl apply -f service.yaml
```
```bash
service.serving.knative.dev/greetings-builtin-code configured
```
```bash
kubectl get ksvc
```
```bash
NAME URL LATESTCREATED LATESTREADY READY REASON
greetings-builtin-code http://greetings-builtin-code.default.example.com greetings-builtin-code-fv6cv greetings-builtin-code-fv6cv True
```
value-data.json
```json
{
"value": {
"name" : "Will",
"place" : "Austin"
}
}
```
Now when curl'ing the endpoint, we expect to see the function run
```bash
curl -d @value-data.json http://greetings-builtin-code.default.example.com
```
instead we see
```json
{"OK":true}
```
I can potentially look into the fix, but I definitely wanted to write down the replication steps before I forgot.

## Environment details:

* local deployment, vagrant, native ubuntu, Mac OS, Bluemix, ...
* version of docker, vagrant, ubuntu, ...

## Steps to reproduce the issue:

1.
2.
3.

## Provide the expected results and outputs:

```
output comes here
```

## Provide the actual results and outputs:

```
output comes here
```

## Additional information you deem important:
* issue happens only occasionally or under certain circumstances
* changes you did or observed in the environment