How do I stub a piece of Express middleware when making an API call with supertest?

I have a route that looks something like this:

app.post( '/api/user/register', user.register, user.doSomething, user.sendPushNotification );

I want to stub user.sendPushNotification so that my application does not actually try to send a push notification.

My user.sendPushNotification method:

exports.sendPairedPushNotification = function( req, res, next ) {
    var user = req.user;

    user.pushNotify( ).then( function(  )
    {
        res.status( 200 ).send( { status: 'sucecss' } );
    } );
};

My test:

var userController = require( '../../server/controllers/user' );
gently.expect( userController, 'sendPushNotification', function( req, res, next )
{
    res.status( 200 ).send( { status: 'success' } );
} );

api.post( '/api/user/register' )
.send( registrationData )
.expect( 200 )
.end( function( error, res )
{
    var User = mongoose.model( 'User' );

    User.find( {  }, function( error, users )
    {
        users.length.should.be.exactly( 1 );
    } );
} );

But this results in an error:

Error: Expected call to [Object].sendPushNotification() did not happen