refactor the algorithm for testing the cors

while here:
- send cors headers even if the status is 404
- add: access_control_expose_headers config option - list of additional headers sent in Access-Control-Expose-Headers
- add: access_control_allow_credentials config option - if true return Access-Control-Allow-Credentials header equal "true"

winixd/core/request.h

@@ -595,7 +595,6 @@ private:
 	int SelectDeflateVersion();
 	void SelectCompression(size_t source_len, bool & compression_allowed, int & compression_encoding);
 	void PrepareSessionCookie();
-	void CheckCorsHeaders();
 	void PrepareHeaders(bool compressing, int compress_encoding, size_t output_size);
 	void ModifyStatusForRedirect();
 	void PrepareSendFileHeaderForStaticMountpoint();