db.create_doc should take the default value of '{}' for content

[Lucio Torre]

With the .content feature the user is no longer required to manually craft any json except in the case of creating docs, where he has to either create a json string to provide to db.create_doc or create a rev_id and doc_id manually to use when creating a document by itself.

It would be easier if we could leave all the initialization stuff to the db class and just use db.create_doc() to get an empty doc that is properly initialized and ready to be used, without having to deal with json or other internals.