Change default stream buffer size to 4096

src/android/src/main/java/com/RNFetchBlob/RNFetchBlobFS.java

                 String eventName = "RNFetchBlobStream+" + path;
                 try {
 
-                    int chunkSize = encoding.equalsIgnoreCase("base64") ? 1026 : 1024;
+                    int chunkSize = encoding.equalsIgnoreCase("base64") ? 4098 : 4096;
                     if(bufferSize > 0)
                         chunkSize = bufferSize;
                     FileInputStream fs = new FileInputStream(new File(path));

src/ios/RNFetchBlob/RNFetchBlob.m

         case NSStreamEventHasBytesAvailable:
         {
             NSMutableData * chunkData = [[NSMutableData data] init];
-            NSInteger chunkSize = 1024;
+            NSInteger chunkSize = 4096;
             if([[self.encoding lowercaseString] isEqualToString:@"base64"])
-                chunkSize = 1026;
+                chunkSize = 4098;
             if(self.bufferSize > 0)
                 chunkSize = self.bufferSize;
             uint8_t buf[chunkSize];