saveUser = function(authdb, doc, callback) {
    var url = '/' + authdb + '/' + doc._id;
    var req = {
        type: 'PUT',
        url: url,
        data: JSON.stringify(doc),
        processData: false,
        contentType: 'application/json'
    };
    db.request(req, callback);
}
Save user to user database.

@name saveUser(authdb, doc, callback)
@param {String} user database
@param {Object} user document
@param {Function} callback
@api private