Add option to force drill to read response body

[asonix]

I'm trying to test an API that serves files, and drill is reporting that the requests are completing much faster than they should, since it makes no attempt to download the response body before ending it's timer.

[fcsonline]

Good to know. It is true that a request is not only the hit but also the transportation step. Right now the download only happens at this point if the request has an assign to be used later.

[acdha]

I'm not sure even assigning a value causes a download in all cases — I hit the same problem for the same reason and adding that option doesn't appear to have changed the timing at all.

[fcsonline]

We need some option to force to read all the response body, but as a workaround assigning the value should read all the content because is used in other use cases like dependencies between requests.