Multi field mapping support for all core type properties

`MultiFieldMapping` needs to support all core type properties in order to preserve and reuse the same `RootObjectMapping` retrieved from a response on another put request.  See #873 for context.  This change will pass the test added in #882.

The better solution is to remove `MultiFieldMapping` and add `Fields` to all core types, which would be a breaking change, thus needs to be done in the 2.0 branch.