RUBY-658 fixing issue with default db from connection uri

We had some broken (or at least it felt broken) logic around default DB names in `MongoClient` when created from a connection string URI that comes from something other than `ENV['MONGODB_URI']` or from `MongoClient#from_uri` directly. This fixes the issue and uses the DB name from the URI before falling back to the 'test' DB default.

In short, this makes the following behave as you'd expect:
```ruby
client = Mongo::MongoClient.from_uri('mongodb://localhost/some_database')
db = client.db
db.name #=> 'some_database'
```

Let's not carry the hard-coded references to `ENV['MONGODB_URI']` over to the 2.0 driver, they need to die here :(