Fix test_preserve_chunked_on_retry flakiness (#1743)
Fix test_preserve_chunked_on_retry flakiness (#1743)

Specifically requesting a retry still tests the same code path, but is
more reliable and will avoid strange broken connections on AppVeyor.

The downside is that the test now takes one second, as we do not allow
fractional Retry-After values.
sigmavirus24/urllib3,urllib3/urllib3,urllib3/urllib3,sigmavirus24/urllib3