http: added 417 response treatment

http: added 417 response treatment

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 #4949
Closes #4964