title: BinaryFormatter not overwriting/updating serialized item
body: <p>I tried using the binaryFormatter as a method of saving and loading all types of data in my game (accounts, score) which are structs containing fields and/or lists. The thing is that it doesn't seem to overwrite or update data in a file once it's serialized. </p>
  
  <p>For example, I create a List containing two PlayerAccounts and serialize it. To be extra careful for testing I deserialize into a new List of Playeraccounts. The first time the data is serialized just fine, but when I add another account to the list and serialize it again, the changes don't seem to be noticed. Maybe I have searched for the issue but it seems I missed out on something. </p>
  
  <p>Because there is a lot of code involved i just created a simple save and load then try to overwrite using a List of ints which does the same thing so I guess it serves as a subsitute.</p>
  
  <p>here's the main code: </p>
  
  <pre><code>string path = Application.persistentDataPath + "/mEs.binary";
  
  FileStream fsWrite = new FileStream(path, FileMode.Append, FileAccess.Write);
  
  BinaryFormatter bf = new BinaryFormatter();
  
  List&lt;int&gt; input = new List&lt;int&gt; { 1, 2, 3, 4 };
  
  bf.Serialize(fsWrite, input);
  
  fsWrite.Close();
  
  FileStream fsRead = new FileStream(path, FileMode.Open, FileAccess.Read);
  
  List&lt;int&gt; output = (List&lt;int&gt;)bf.Deserialize(fsRead);
  
  fsRead.Close();
  
  print("first entry:");
  
  foreach (int entry in output)
  {
      print(entry);
      //prints 1 to 4
  }
  
  input = output;
  
  input.Add(5);
  
  fsWrite = new FileStream(path, FileMode.Append, FileAccess.Write);
  
  bf.Serialize(fsWrite, input); //add the updated List of ints
  
  fsWrite.Close();
  
  fsRead = new FileStream(path, FileMode.Open, FileAccess.Read);
  
  output = (List&lt;int&gt;)bf.Deserialize(fsRead);
  
  print("second entry:");
  
  foreach (int entry in output)
  {
      print(entry);
      //prints 1 to 4 again (instead of 1 to 5)
  }
  </code></pre>