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.

AUTHORS

@@ -237,3 +237,4 @@ that much better:
  * Bryan Bennett (https://github.com/bbenne10)
  * Gilb's Gilb's (https://github.com/gilbsgilbs)
  * Joshua Nedrud (https://github.com/Neurostack)
+ * Shu Shen (https://github.com/shushen)