Fix issue with default route prefix

[MGudgin]

If the Azure Functions app used the default route prefix of api, but
that route prefix was NOT specified in host.json, the GetHostRoutePrefix
method would return null. This would break local debugging as the
ExecuteFunction implementation would try to invoke the wrong URI for
the target function. For example, it would try to invoke /Test rather
than /api/Test.

This PR fixes this issue by having GetHostRoutePrefix return 'api' in
the case where no route prefix is specified in host.json.

This change has been tested with four cases;

1. Default route prefix, no prefix specified in host.json.
2. Default route prefix, prefix specified in host.json.
3. Custom, non-empty route prefix, prefix specified in host.json.
4. Custom, empty route prefix, prefix specified in host.json.

Also update readme.md to call out that custom route prefixes require
changes to playfab.local.settings.json.