Match the Expect 100-continue expectation as a token

RFC 9110 Section 10.1.1 defines Expect as a comma-separated list, states that
its value is case-insensitive, and requires a server that receives a
100-continue expectation in an HTTP/1.0 request to ignore it. Comparing the
whole field value against "100-continue" met none of those.

An HTTP/1.0 request asking for 100-continue was answered with a 100 (Continue)
interim response, which that section forbids. "100-Continue" and
"100-continue, foo" were both read as no expectation at all, so a client that
waits for the interim response before sending its content waited for a response
that was never coming.

Route the check through has_header_token(), which walks every field line and
compares complete tokens case-insensitively, and skip it for HTTP/1.0. An
expectation cpp-httplib does not recognize is still ignored rather than
refused; the 417 the section offers for one is a MAY, not a requirement.

httplib.h

@@ -13519,7 +13519,12 @@ Server::process_request(Stream &strm, const std::string &remote_addr,
 
   if (setup_request) { setup_request(req); }
 
-  if (req.get_header_value("Expect") == "100-continue") {
+  // RFC 9110 10.1.1: Expect is a comma-separated list whose value is
+  // case-insensitive, and a 100-continue expectation in an HTTP/1.0 request
+  // must be ignored. An expectation we do not recognize is left alone; the
+  // 417 the section allows for one is a MAY, not a requirement.
+  if (req.version != "HTTP/1.0" &&
+      detail::has_header_token(req.headers, "Expect", "100-continue")) {
     int status = StatusCode::Continue_100;
     if (expect_100_continue_handler_) {
       status = expect_100_continue_handler_(req, res);
@@ -15082,7 +15087,8 @@ inline bool ClientImpl::process_request(Stream &strm, Request &req,
   }
 
   // Check for Expect: 100-continue
-  auto expect_100_continue = req.get_header_value("Expect") == "100-continue";
+  auto expect_100_continue =
+      detail::has_header_token(req.headers, "Expect", "100-continue");
 
   // Send request (skip body if using Expect: 100-continue)
   auto write_request_success =