Fixed amibiguity and differing behaviour regarding field defaults (#349)

Now field defaults are king, unsetting or setting to None on a field
with a default means the default is reapplied.

mongoengine/document.py

@@ -206,7 +206,7 @@ class Document(BaseDocument):
         if validate:
             self.validate(clean=clean)
 
-        if not write_concern:
+        if write_concern is None:
             write_concern = {"w": 1}
 
         doc = self.to_mongo()