How to invoke a web action for .json context extension via wsk command?

apache/openwhisk#4712 Environment details:

local deployment

## Steps to reproduce the issue:

1. make a simple raw HTTP web action in python
```
def main(args):
val = args['__ow_headers']
print(args)
return { "response": args,
"val": val }
```

2. try to invoke it with wsk command
```
$ wsk action invoke -r cs/jin-test
{
"error": "The action did not return a dictionary."
}
```

3. check out the activation log
```
wsk activation get --last
"logs": [
"2019-12-06T04:11:54.832861208Z stderr: Traceback (most recent call last):",
"2019-12-06T04:11:54.832914474Z stderr: File \"pythonrunner.py\", line 88, in run",
"2019-12-06T04:11:54.832926486Z stderr: exec('fun = %s(param)' % self.mainFn, self.global_context)",
"2019-12-06T04:11:54.832934019Z stderr: File \"\", line 1, in ",
"2019-12-06T04:11:54.832941Z stderr: File \"__main__.py\", line 3, in main",
"2019-12-06T04:11:54.833038393Z stderr: KeyError: '__ow_headers'"
],
```

4. But it works with curl when invoked with .json context extension
```
curl https://frisb.kakao.com/api/v1/web/jin.c/cs/jin-test.json
{
"response": {
"__ow_body": "",
"__ow_headers": {
"accept": "*/*",
"host": "controllers",
"user-agent": "curl/7.54.0",
"x-request-id": "e7e3d9a52c099c6a6e46bce11207d5a4"
},
"__ow_method": "get",
"__ow_path": "",
"__ow_query": ""
},
"val": {
"accept": "*/*",
"host": "controllers",
"user-agent": "curl/7.54.0",
"x-request-id": "e7e3d9a52c099c6a6e46bce11207d5a4"
}
```

## What I want

I'd love to know if there is a way for the wsk command to invoke a web action with context extensions. I learned that the default is .http in the docs and I wonder if there is no way for wsk to invoke an action other context extensions but .http.