Improve mapping of nested arrays.

This fixes 2 bugs I met when using mongoosastic with nested arrays.

1. extra ES attributes (like es_index) were ignored when applied to the simple type content of an array.

2. mappings weren't generated for arrays with complex objects.

I think it is related to this issue: https://github.com/jamescarr/mongoosastic/issues/65

And maybe a few more.