Update ReactNativeBlobUtil.mm

ensuring readfile calls with ascii encoder options don't fall through the utf8 else case

Author: Daniel528

ios/ReactNativeBlobUtil/ReactNativeBlobUtil.mm

@@ -680,8 +680,7 @@ - (void)readFile:(NSString *)path
         }
         if([encoding isEqualToString:@"ascii"]) {
             resolve((NSMutableArray *)content);
-        }
-        if([encoding isEqualToString:@"base64"]) {
+        } else if([encoding isEqualToString:@"base64"]) {
             resolve([content base64EncodedStringWithOptions:0]);
         } else {
             resolve([[NSString alloc] initWithData:content encoding:NSUTF8StringEncoding]);