When passed a filename, download a report in chunks

Rather than pulling an entire 6-8 GB report into a string in memory to
immediately write it out to a file, use the built-in feature of Net::HTTP to
retrieve the result in chunks and write them out as they are read from the
remote host. This prevents receiving `NoMemoryError` issues when downloading
huge reports.

lib/nexpose/connection.rb

@@ -143,11 +143,15 @@ def download(url, file_name = nil)
         http.cert_store = @trust_store
       end
       headers = { 'Cookie' => "nexposeCCSessionID=#{@session_id}" }
-      resp    = http.get(uri.to_s, headers)
 
       if file_name
-        ::File.open(file_name, 'wb') { |file| file.write(resp.body) }
+        http.request_get(uri.to_s, headers) do |resp|
+          ::File.open(file_name, 'wb') do |file|
+            resp.read_body { |chunk| file.write(chunk) }
+          end
+        end
       else
+        resp = http.get(uri.to_s, headers)
         resp.body
       end
     end