Different promise implementations are not accepted

[jrvidal]

If I try to do something like:

res.send(query.exec().then(function(data) {
    var formattedData = format(data);
    return data;
}));

the plugin does not work, since it is checking for the mongoose promise implementation in particular, and the then method returns a MPromise. It would be nice to be able to interoperate with thenables in general or, at least, to redefine mongoose.Promise#then to return mongoose.Promise instances.

[aheckmann]

instead of using instanceof checks for Promises we could add an isPromise() function which accepts an arg and checks if 'function' == typeof arg.then. pull request?