Closes marshmallow-code#95: fields2jsonschema warning about Shema Object

Add extra check to show warning about fields ignoring only if there are some of them

apispec/ext/marshmallow/swagger.py

@@ -453,7 +453,10 @@ class Meta:
     :rtype: dict, a JSON Schema Object
     """
     Meta = getattr(schema, 'Meta', None)
-    if getattr(Meta, 'fields', None) or getattr(Meta, 'additional', None):
+    declared_fields = set(schema._declared_fields.keys())
+    if (set(getattr(Meta, 'fields', set())) > declared_fields
+        or
+        set(getattr(Meta, 'additional', set())) > declared_fields):
         warnings.warn(
             "Only explicitly-declared fields will be included in the Schema Object. "
             "Fields defined in Meta.fields or Meta.additional are ignored."