Wrong elapsed timedelta #1310

Checklist

• [ X] The bug is reproducible against the latest release and/or master.
• [ X] There are no similar issues or pull requests to fix it yet.

Describe the bug

The elapsed argument obtained following a stream request seems completely incoherent.

To reproduce

Code example with httpx 0.15.1:

import httpx

client = httpx.Client()

with client.stream("GET", "https://httpbin.org/basic-auth/user/pass", auth=("user", "pass")) as r:
   print(r)

print(r.elapsed)

Expected behavior

Result got with httpx 0.14.3: 0:00:02.275297

Actual behavior

Result got with httpx 0.15.1: 1 day, 17:07:15.687523

Debugging material

Environment

• OS: Linux
• Python version: 3.7.5
• HTTPX version: 0.15.1
• Async environment: None
• HTTP proxy: No
• Custom certificates: No

Thanks,