Allow autoupdate:false as a valid parameter when configuring a Mango index

## Summary

The Mango validation currently rejects this flag, but we should allow it as it has meaning in the server.

## Desired Behaviour

A POST to `_index` can create an index that is not updated automatically without manually tweaking the design document.

Start by tracing Mango validation for POST requests to the _index endpoint. Check how the autoupdate parameter is validated, then verify that autoupdate:false is accepted and creates an index that is not updated automatically without manual design-document changes.

Written by the indexing model from the issue text.