Introduce limits middleware

1. Replace original `maxrequestbody` directive.
2. Add request header limit.

fix issue #1587

Signed-off-by: Tw <tw19881113@gmail.com>

caddyhttp/httpserver/replacer.go

@@ -302,7 +302,7 @@ func (r *replacer) getSubstitution(key string) string {
 		}
 		_, err := ioutil.ReadAll(r.request.Body)
 		if err != nil {
-			if _, ok := err.(MaxBytesExceeded); ok {
+			if err == MaxBytesExceededErr {
 				return r.emptyValue
 			}
 		}