100 Continue
https://httpfox.gay/100
Description
The HTTP 100 Continue informational status response code indicates that everything so far is OK
and that the client should continue with the request or ignore it if it is already finished.
To have a server check the request's headers, a client must send Expect: 100-continue as a
header in its initial request
and receive a 100 Continue status code in response before sending the body.