Download time of response body is not included in Response.elapsed

When doing a request with stream=False, to an endpoint that responds with a streaming/chunked response, the download time of the response body is not included in the Response.elapsed timedelta.

This pull request adds a test for this that currently fails.

In Requests 2.2 the behaviour was different, and this test passes. If the current behaviour is the intended, I guess one could just modify the test in this pull request to reflect the expected behaviour.