connect() does not use connection settings previously registered as default

Hi,

I use register_connection() to register the default connection settings and connect() to establish a connection afterwards. When calling connect() without an alias, I would expect to connect using the default settings previously defined with register_connection(), though connect() ignores the default alias and falls back to the sane defaults:

>>> import mongoengine as m
>>> m.register_connection('default', db='mongoenginetest', host='mongomock://user:pass@localhost1/mongoenginetest')
>>> conn = m.connect(db='mongoenginetest')
>>> conn
MongoClient(host=['localhost:27017'], document_class=dict, tz_aware=False, connect=True, read_preference=Primary())

Note, the host used when registering the default alias and the host of the established client are different. connect() ends up overwriting the default connection settings.

get_connection() seems to respect the default connection settings instead:

>>> import mongoengine as m
>>> m.register_connection('default', db='mongoenginetest', host='mongomock://user:pass@localhost1/mongoenginetest')
>>> conn = m.connection.get_connection()
>>> conn
mongomock.MongoClient('mongodb://user:pass@localhost1/mongoenginetest', 27017)

I am a bit confused whether this is an expected behaviour or not. My proposed fix is changing connect() to look at _connection_settings instead of _connections before registering a connection which doesn't seem to break any of the existing tests.

Start by reading the connect(), register_connection(), and get_connection() entry points, then reproduce the issue using the default alias and the _connection_settings and _connections state described here. Done means connect() preserves the previously registered default settings, including the host, with tests covering the behavior.