utils: seek @ from tail
User can set a password that contains '@'.
If we seek '@' from head to get proxy host, we will fail to split password.
e.g.
The password should be  'example_pass_w_@_char'.
@@ -1037,7 +1037,7 @@ int flb_utils_proxy_url_split(const char *in_url, char **out_protocol,
proto_sep += 3;
/* Seperate `username:password` and `host:port` */
- at_sep = strchr(proto_sep, '@');
+ at_sep = strrchr(proto_sep, '@');
if (at_sep) {
/* Parse username:passwrod part. */
tmp = strchr(proto_sep, ':');
