Fix AttributeError when creating EmbeddedDocument

When an EmbeddedDocument is initialized with positional arguments, the
document attempts to read _auto_id_field attribute which may not exist
and would throw an AttributeError exception and fail the initialization.

This change and the test is based on the discussion in issue #681 and
PR #777 with a number of community members.

mongoengine/base/document.py

@@ -51,7 +51,7 @@ class BaseDocument(object):
             # We only want named arguments.
             field = iter(self._fields_ordered)
             # If its an automatic id field then skip to the first defined field
-            if self._auto_id_field:
+            if getattr(self, '_auto_id_field', False):
                 next(field)
             for value in args:
                 name = next(field)