Fallback to signatureCipher if needed

I cannot replicate this myself, however, some users have reported page requests that contain `signatureCipher` instead of `cipher`.  See ytdl-org/youtube-dl#25188

Note the page dump at https://pastebin.com/m6dRwg1f is in base64 and needs to be decoded to search for the `signatureCipher` keys.

XCDYouTubeKit/XCDYouTubeVideo.m

@@ -397,7 +397,7 @@ - (instancetype) initWithIdentifier:(NSString *)identifier info:(NSDictionary *)
 		{
 			stream = XCDDictionaryWithQueryString((NSString *)streamQuery);
 		}
-		NSDictionary *alternativeStreamInfo = XCDDictionaryWithQueryString(stream[@"cipher"]);
+		NSDictionary *alternativeStreamInfo = XCDDictionaryWithQueryString(stream[@"cipher"]) == nil? XCDDictionaryWithQueryString(stream[@"signatureCipher"]) : XCDDictionaryWithQueryString(stream[@"cipher"]);
 		NSString *alternativeURLString = alternativeStreamInfo[@"url"];
 
 		NSString *scrambledSignature = stream[@"s"] == nil? alternativeStreamInfo[@"s"] : stream[@"s"];