Fix for missing response headers

Hi, I was testing a Moose based wrapper of Net::CampaignMonitor yesterday and the headers field of the results was always a small positive integer. Digging a bit I found that REST::Client's headers were not being obtained properly.

In the process of fixing this I removed a lot of repetition in the code by introducing a private method, _build_results().

The wrapper I'm writing is called Net::CampaignMonitor::Moosey. It has already proved useful, if I publish it do you mind me taking this part of the CPAN namespace?

I forgot to add a test for the response headers so I recreated the pull request.

Cheers,
Andrew Kirkpatrick (University of Adelaide Marketing Dept)