Fix unrecognized selector error

Summary:
Fixes "[NSTaggedPointerString unsignedIntValue]: unrecognized selector" error
Closes #15755

Differential Revision: D5763628

Pulled By: shergin

fbshipit-source-id: 6e1d1b1ae6bc63e07112b9dddcdf8e6f812a7f51

Author: giladno

React/Base/RCTMultipartStreamReader.m

@@ -76,7 +76,7 @@ - (void)emitProgress:(NSDictionary *)headers
   // Throttle progress events so we don't send more that around 60 per second.
   CFTimeInterval currentTime = CACurrentMediaTime();
 
-  NSUInteger headersContentLength = headers[@"Content-Length"] != nil ? [headers[@"Content-Length"] unsignedIntValue] : 0;
+  NSInteger headersContentLength = headers[@"Content-Length"] != nil ? [headers[@"Content-Length"] intValue] : 0;
   if (callback && (currentTime - _lastDownloadProgress > 0.016 || final)) {
     _lastDownloadProgress = currentTime;
     callback(headers, @(headersContentLength), @(contentLength));