Change session validation to a case-sensitive one

It's possible to join servers with different capitalizations of your username, always having different inventories, spawns, etc.

What I expected to happen was...:
I won't be able to join any servers, due to session validation problems.

What actually happened was...:
It worked just fine, I had a different inventory, different spawns, etc. like I have had just joined with a new account.

Steps to Reproduce:

• Join a server as usual, gather some material/get some levels/anything else
• Leave the server
• Use the following code somewhere AFTER creating your session.
  Session session = Minecraft.getMinecraft().session;
  session.username = session.username.toUpperCase();
• Login into any server

Update since 1.6: As far as I have seen the fields are now private and final, so you need to do some reflections to achieve the same result.

I haven't looked into any code regarding the session validation yet but this has most likely something to do with your servers, checking the session-username case-insensitive.