Endpoints should return 308 status rather than 301 on POST requests

Currently, the api issues a 301 response if a request arrives with a `http` scheme as opposed to an `https` scheme. The 301 status works fine for `GET` requests and forwards the request correctly, but for some user-agents a 301 status leads to a `POST` request being changed to a `GET` request.

A `308` response serves the same purpose as a `301`, but leaves the method of the request intact, meaning that the `POST` request is served correctly. Here's a cURL request that you can try out:
```curl
curl -L -X POST -H "apca-api-key-id: KEY_ID_HERE" -H "apca-api-secret-key: SECRET_KEY_HERE" -H "Content-Type: application/json" --data '{"symbol":"V","qty":"1","side":"buy","type":"market","time_in_force":"gtc"} http://paper-api.alpaca.markets/v2/orders'
```
Because of the 301 status, the POST method in that request gets clobbered into a GET method after the redirect and we get an error