Fix string comparison in SetCurrentDrive

I've explored the case when current path is `d:\Something\Something` (well, I have this setup in my debug environment). In this case, drive letter returned from `System.Management.Path.GetPath` is lowercase (i.e. `"d"`). But letters returned from FileSystemProvider are all uppercase (i.e. `"D"`). Because of that, an exception `"why are we here?"` is thrown.

So I've decided to better fix `SessionStateGlobal.SetCurrentDrive` code, because I think that we should never do assumptions about drive letter casing.