When doing a request with a body + Expect: 100-continue and the server
responds with a 417, the same request will be retried immediately
without the Expect: header.
Added test 357 to verify.
Also added a control instruction to tell the sws test server to not read
the request body if Expect: is present, which the new test 357 uses.
Reported-by: bramus on github
Fixes#4949Closes#4964
