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 aospan@netup.ru