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.

docs/changelog.rst

@@ -12,6 +12,7 @@ Changes in 0.10.7 - DEV
 - Fixed long fields stored as int32 in Python 3. #1253
 - MapField now handles unicodes keys correctly. #1267
 - ListField now handles negative indicies correctly. #1270
+- Fixed AttributeError when initializing EmbeddedDocument with positional args. #681
 
 Changes in 0.10.6
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