allow type name to be overridden when adding mappings at index creation

[nordbergm]

This change allows you to use the PutMappingDescriptor's existing Type method to override the type name of a mapping at index creation.

Since it was already supported for PutMapping, having it at index creation too made sense.

The use case is pretty simple. If you have a base class which you want to either use attribute mappings for, or fluent mapping, you could put that base class mapping as "default" by doing something like:

_client.CreateIndex("index",
                    c =>
                    c.AddMapping<Entity>(
                        map =>
                            map.Type("_default_")
                               .Properties(p => p.String(s => s.Name(n => n.Name)
                                                           .Index(FieldIndexOption.analyzed)
                                                           .Store(false)
                                                           .IncludeInAll()))
                               .DynamicTemplates(d => d.Add(a =>
                                                            a.Name("other")
                                                             .Match("*")
                                                             .Mapping(m => m.Generic(g => g.Index("not_analyzed")
                                                                                           .IncludeInAll(false)))))))

[Mpdreamz]

Thanks @nordbergm seems rather silly this wasn't already supported :)