Add db parameter to register_connection

This is done to make it compatible with the connect function.

docs/changelog.rst

@@ -9,6 +9,7 @@ dev
 -  Improve LazyReferenceField and GenericLazyReferenceField with nested fields #1704
 -  Fix validation error instance in GenericEmbeddedDocumentField #1067
 -  Update cached fields when fields argument is given #1712
+-  Add a db parameter to register_connection for compatibility with connect
 
 Changes in 0.15.0
 =================

mongoengine/connection.py

@@ -28,7 +28,7 @@ _connections = {}
 _dbs = {}
 
 
-def register_connection(alias, name=None, host=None, port=None,
+def register_connection(alias, db=None, name=None, host=None, port=None,
                         read_preference=READ_PREFERENCE,
                         username=None, password=None,
                         authentication_source=None,
@@ -39,6 +39,7 @@ def register_connection(alias, name=None, host=None, port=None,
     :param alias: the name that will be used to refer to this connection
         throughout MongoEngine
     :param name: the name of the specific database to use
+    :param db: the name of the database to use, for compatibility with connect
     :param host: the host name of the :program:`mongod` instance to connect to
     :param port: the port that the :program:`mongod` instance is running on
     :param read_preference: The read preference for the collection
@@ -58,7 +59,7 @@ def register_connection(alias, name=None, host=None, port=None,
     .. versionchanged:: 0.10.6 - added mongomock support
     """
     conn_settings = {
-        'name': name or 'test',
+        'name': name or db or 'test',
         'host': host or 'localhost',
         'port': port or 27017,
         'read_preference': read_preference,