Fix embedded doc exception with new mongo-cxx

[aospan]

With modern mongo-cxx (tested on 3.3.1) we have exception while using embedded documents:

terminate called after throwing an instance of
'bsoncxx::v_noabi::exception'
what(): can't convert builder to a valid view: unmatched key

For example: { "name" : "Jenny", "contact_info" : { "type" : "home" } }

we have called twice:

1. for "contact_info" key
2. for "contact_info.type" key

we can't call key_view/key_owned twice. Otherwise we receive exception as described above.

Signed-off-by: Abylay Ospan [email protected]