The 1xx (Informational) class of status code indicates an interim
response for communicating connection status or request progress
prior to completing the requested action and sending a final
response. 1xx responses are terminated by the first empty line after
the status-line (the empty line signaling the end of the header
section). Since HTTP/1.0 did not define any 1xx status codes, a
server MUST NOT send a 1xx response to an HTTP/1.0 client.
A client MUST be able to parse one or more 1xx responses received
prior to a final response, even if the client does not expect one. A
user agent MAY ignore unexpected 1xx responses.
A proxy MUST forward 1xx responses unless the proxy itself requested
the generation of the 1xx response. For example, if a proxy adds an
“Expect: 100-continue” field when it forwards a request, then it need
not forward the corresponding 100 (Continue) response(s).