Promisify if it calls a callback, pass through if it returns a promise.

maybePromisify.js

// non-`Promise.defer()` version of https://github.com/petkaantonov/bluebird/issues/159#issuecomment-38348877

var Promise = require('bluebird');

function maybePromisify(fn, ctx){
    return function() {
        var args = Array.prototype.slice.call(arguments);
        return new Promise(function(fulfill, reject){
            args.push(function(err, result){
                if (err)
                    reject(err);
                else 
                    fulfill(result);
            });
            var p = fn.apply(ctx, args);
            if (p && typeof p.then === 'function')
                fulfill(p);
        });
    }
}