[BUG][Python-Flask] camelCase path parameters are not converted to snake_case

Created by: tomghyselinck

Description

We have parameterized paths in our OpenAPI v3.x definition file.
We generate a and start python-flask server.

When trying to obtain the data, we get an internal server error and the server logs:

[2018-12-14 10:20:05,725] ERROR in app: Exception on /data/123 [GET]
Traceback (most recent call last):
  File "/home/tom/.local/lib/python3.5/site-packages/flask/app.py", line 2292, in wsgi_app
    response = self.full_dispatch_request()
...
  File "/home/tom/.local/lib/python3.5/site-packages/connexion/decorators/decorator.py", line 44, in wrapper
    response = function(request)
  File "/home/tom/.local/lib/python3.5/site-packages/connexion/decorators/parameter.py", line 131, in wrapper
    return function(**kwargs)
TypeError: data_remote_id_get() got an unexpected keyword argument 'remoteId'
127.0.0.1 - - [14/Dec/2018 10:20:05] "GET /data/123 HTTP/1.1" 500 -

The camelCase path parameter is not converted to a snake_case (== pythonic) function argument for the controller.

openapi-generator version

I used OpenAPI generator CLI version 4.0.0-SNAPSHOT: https://oss.sonatype.org/content/repositories/snapshots/org/openapitools/openapi-generator-cli/4.0.0-SNAPSHOT/openapi-generator-cli-4.0.0-20181210.103357-85.jar

OpenAPI declaration file content or url

See python-flask-pythonic-params.yaml in the attached zip-file: python-flask-pythonic-params.zip

Command line used for generation

java -jar openapi-generator-cli-4.x.jar generate -i ./python-flask-pythonic-params.yaml -g python-flask -o ./python-flask-pythonic-params/

Steps to reproduce

Generate the server code
```
./python-flask-pythonic-params.sh
```

Start the server

(cd python-flask-pythonic-params && python3 -m openapi_server)

Perform a client request

curl -X GET "http://localhost:8080/data/123" -H  "accept: application/json"

Returns:

{
  "detail": "The server encountered an internal error and was unable to complete your request.  Either the server is overloaded or there is an error in the application.",
  "status": 500,
  "title": "Internal Server Error",
  "type": "about:blank"
}

Related issues/PRs

#1856

Suggest a fix

Some suggestions (feel free for other suggestions ;-)):

Define **kwargs as argument in the controllers and get the (non-pythonic) argument via kwargs['remoteId'].
Provide pythonic_params=True to the connexion/Flask app. I verified that this works.

In our example this would be:

Replacing:
```
app.add_api('openapi.yaml', arguments={'title': 'Test REST API'})
```
by
```
app.add_api('openapi.yaml', arguments={'title': 'Test REST API'}, pythonic_params=True)
```
See also zalando/connexion:
- #687 - pythonic_params does not cover the path params