bug#34855: 27.0.50; url-retrieve-synchronously Bad request Error every 2nd call

[Eli Zaretskii]

> From: Thomas Hisch <thomas.hisch@ims.co.at>
> Date: Thu, 14 Mar 2019 12:01:54 +0100
> 
> The emacs 27.0 build, where the test fails,  was build be me locally.
> Could it be that something between emacs 26.1 and the HEAD of master
> broke sth?

Please see if it weren't my recent changes in commit 9486d87cfe that
broke this.

If my changes are not the reason, I'd appreciate if you could bisect
to find the commit which broke this, since the reproduction recipe you
provided is quite complex.  Alternatively, a simpler reproducer
starting from "emacs -Q" would be welcome.

Thanks.