David Wright filed bug report #1849764

(http://curl.haxx.se/bug/view.cgi?id=1849764) with an included fix. He
identified a problem for re-used connections that previously had sent
Expect: 100-continue and in some situations the subsequent POST (that didn't
use Expect:) still had the internal flag set for its use. David's fix (that
makes the setting of the flag in every single request unconditionally) is
fine and is now used!