Keep header 'Accept' values 'application/json'

I debug the code for solving my issue ＃24（https://github.com/gondor/openstack4j/issues/24） 
I found that when I call ```List<? extends Meter> meters= meterService.list();```,the code in method invoke() here entity is null, and  the code runs the lase return ```return HttpResponse.wrap(invocation.method(request.getMethod().name()));```
so I add the invocation's Accpet Type by add MediaType.APPLICATION_JSON to the target.request() method to make sure the request header 'Accept' always values 'application/json'
Merge pull request #25 from xyfigo/patch-1

Keep header 'Accept' values 'application/json'