[SalesForce] how to test a future method which makes callouts

I am practising the Integration workbook provided by salesforce. In tutorial#2 there's a class which makes future callouts to a Heroku app.This class is called by an after update trigger.
Callout class:

public class UpdateHeroku {
 public class ExternalOrder{
        public string id{get;set;}
        public Integer order_number{get;set;}
    } 

    @future(callout=true)
    public static void postOrder(list<id> invoiceids){
        JSONGenerator gen = Json.createGenerator(true);
        gen.writeStartArray();
        for(id invoiceid:invoiceids){
            gen.writeStartObject();
            gen.writeStringField('id', invoiceid);
            gen.writeEndObject();
        }
        gen.writeEndArray();

        string jsonOrders = gen.getAsString();
        System.debug('jsonOrders: '+ jsonOrders);

        HTTPrequest req = new HTTPrequest();
        req.setMethod('POST');
        req.setEndpoint('https://dry-springs-6303.herokuapp.com');
        req.setHeader('Content-Type', 'application/json');
        req.setBody(jsonOrders);
        req.setTimeout(30000);

        Http http = new Http(); 
        HttpResponse res = http.send(req);
        System.debug('Fulfillment service returned '+ res.getBody());

        if(res.getStatusCode()!=200){
            System.debug('Error from '+ req.getEndpoint() + ':'+ res.getStatusCode() + ' ' + res.getStatus());
        }
        else
        {
            list<invoice__c> invoices = [select id from Invoice__c where id in :invoiceids];
            list<externalorder> orders = (list<ExternalOrder>)JSON.deserialize(res.getBody(), list<ExternalOrder>.class);

        Map<Id, Invoice__c> invoiceMap = new Map<Id, Invoice__c>(invoices);
        for(externalorder order:orders){
            Invoice__c invoice = invoiceMap.get(order.id);
            invoice.orderid__c = String.valueOf(order.order_number);
            }
            update invoices;
        } 
    }
}

I have written a test class for trigger which covers 62% of the callout class. I am figuring a way to separately unit test the callout class & wrote an implementation of mock interface

Test Class- which lead to code coverage of 66%

 @isTest
    public class UpdateHeroku_Test {
        @isTest static void testCallout(){
            Test.setMock(HttpcalloutMock.class, new MockGenerator_Test());

            list<invoice__c> invoices = new list<invoice__c>();
            list<id> invoiceids = new list<id>();
            invoices.add(new invoice__c(status__c='Negotiating'));
            invoices.add(new invoice__c(status__c='Pending'));
            insert invoices;
            invoices[0].status__c = 'Closed';
            invoices[1].status__c = 'Closed';
            update invoices;
            invoiceids.add(invoices[0].id);
            invoiceids.add(invoices[1].id);
            Test.startTest();
            UpdateHeroku.postOrder(invoiceids);
            Test.stopTest();
            System.assertEquals(expected1, invoice[0].orderid__c);
            System.assertEquals(expected2, invoice[1].orderid__c);
        }

global class MockGenerator_Test implements HttpCalloutMock{
        global httpResponse respond(HTTPrequest req){

            System.assertEquals('https://dry-springs-6303.herokuapp.com', req.getEndpoint());
            System.assertEquals('POST', req.getMethod());

            HttpResponse res = new HttpResponse();
            res.setHeader('Content-Type', 'application/json');
            res.setStatusCode(405);
            res.setStatus('Method Not Allowed');
            return res;
        }
      }

}

Since future methods can only return void type. I can't verify values received in HttpResponse using assertEquals method. Something like below which I intend to –

String contentType = res.getHeader('Content-Type');
System.assert(contentType == 'application/json');
String actualValue = res.getBody();
String expectedValue = '{"foo":"bar"}';
System.assertEquals(actualValue, expectedValue);

How can I verify the response returned? Thanks,