supporting response docs for mongo 2.2+

Hello :)

I was trying to login to mongo versions 2.2 and 2.4 without success. What I found is that the response on those versions have many docs (more than version 2.0), so the <<"ok">> msg could not be found. Example:

For 2.0, we get:

```erlang
[
  [{<<"ok">>,1.0}]
]
```

For 2.2 and 2.4, we get:

```erlang
[[
  {<<"dbname">>,<<"blog">>},
  {<<"user">>,<<"root">>},
  {<<"readOnly">>,false},
  {<<"ok">>,1.0}
]]
```

The fix will not assume that we get only one element in the response list, tested with 2.0, 2.2, and 2.4, so I hope it is acceptable.

Cheers,