Fix --http-allow-header to filter non existent headers

Fix https://github.com/buger/goreplay/issues/604

http_modifier.go

@@ -98,7 +98,11 @@ func (m *HTTPModifier) Rewrite(payload []byte) (response []byte) {
 		for _, f := range m.config.headerFilters {
 			value := proto.Header(payload, f.name)
 
-			if len(value) > 0 && !f.regexp.Match(value) {
+			if len(value) == 0 {
+                return
+            }
+
+            if !f.regexp.Match(value) {
 				return
 			}
 		}