Fix File.expand_path behavior

This script:

```ruby
require 'fakefs/safe'

puts File.expand_path('directory', '~/parent')
FakeFS.activate!
puts File.expand_path('directory', '~/parent')
FakeFS.deactivate!
```

prints different results for the two expand_path calls, when they should be the
same. If anyone knows a possible reason, let me know! I'm going to start debugging.