Availability of content search not indicated correctly in capabilities

## Describe the bug
The availability of content search is not correctly indicated in the server's capabilities if content search is in fact not available.

## Steps to reproduce
1. Log in at `demo.owncloud.com`
2. Check capabilities `ocs.data.capabilities.search.property` for `content` and it to be `enabled` - which is the case.
3. Upload file with keyword in content
4. Wait a minute to give the server time for indexing
5. Search for keyword in content

## Expected behavior
File is found.

## Actual behavior
The file is not found.

## Additional context
The property at step 2 should indicate whether content search is actually available - either by omitting `content` in `ocs.data.capabilities.search.property` - or by setting its `enabled` value to `false`.

This issue was found during QA of [server-side search in the iOS app](https://github.com/owncloud/ios-app/pull/1419).