Ignore an SSE retry field that is not all digits (#2591)

parse_sse_line checked only the error code of from_chars, which accepts
a leading '-' and stops at the first non-digit, so retry: -1 made the
client reconnect without waiting and retry: 10s set 10 ms. The SSE spec
ignores a retry value that is not all ASCII digits.

httplib.h

@@ -4914,7 +4914,8 @@ inline bool SSEClient::parse_sse_line(const std::string &line, SSEMessage &msg,
     msg.id = value;
   } else if (field == "retry") {
     // Parse retry interval in milliseconds
-    {
+    // Per the SSE spec, a value that is not all ASCII digits is ignored.
+    if (detail::is_numeric(value)) {
       int v = 0;
       auto res =
           detail::from_chars(value.data(), value.data() + value.size(), v);