forgot to implement PIHTTPClient headers

libs/main/http_client/pihttpclient.h

@@ -3,6 +3,7 @@
 
 #include "pihttptypes.h"
 #include "pip_http_client_export.h"
+#include "pistringlist.h"
 
 
 class PIHTTPClientBase {
@@ -50,6 +51,7 @@ private:
 
 	PIString url;
 	PIString last_error;
+	PIStringList headers;
 	PIByteArray buffer_out;
 	PIHTTP::MessageMutable request, reply;
 	std::atomic_bool is_cancel = {false};