POST systemkey should return 409 if resource is already existed

#### Expected behavior

POST http://myApp.azurewebsites.net/admin/host/systemkeys/Key1?code=adminKey
would return HTTP 409 if `Key1` already existed: [a stackoverflow answer on using POST for creation](https://stackoverflow.com/questions/3825990/http-response-code-for-post-when-resource-already-exists)

#### Actual behavior

it overrides existing systemkey with a new value

I feel like overriding a systemkey is pretty significant, since it invalidates previously created authenticated URLs, if we still want the ability to update a systemkey with a non-user specified value, we would expand the PUT API
https://github.com/Azure/azure-webjobs-sdk-script/blob/56d2014240e8f7d577c81dd723fead0030ca95eb/src/WebJobs.Script.WebHost/Controllers/KeysController.cs#L103-L113

Read KeysController.cs around lines 103-113 and trace the POST systemkey path for an existing Key1. Confirm the desired behavior is a 409 response without replacing the current value, while preserving creation of new keys. The issue does not name a test file, so locate the existing controller coverage before verifying both cases.

Written by the indexing model from the issue text.