Currently the put-mapping API assumes that because the type name is `_doc` then
it is dealing with a typeless put-mapping call. Yet we still allow running the
put-mapping API in a typed fashion with `_doc` as a type name. The current logic
triggers surprising errors when doing a typed put-mapping call with `_doc` as a
type name on an index that has a type already.

This is a bit of a corner-case, but is more important on 6.x due to the fact
that using the index API with `_doc` as a type name triggers typed calls to the
put-mapping API with `_doc` as a type name.