data-differ does not support _ids that are dicts

[dbonser]

Document: { "_id" : { "key1" : "value1", "key2" : "value2" } } data-differ fails with: An error occurred while comparing documents: unhashable type: 'dict'

Looks like the _id dict could be handled as described at https://stackoverflow.com/questions/13264511/typeerror-unhashable-type-dict