composer stores vendor dir in `$XDG_CONFIG_HOME`

I seem to have a vendor directory with all source files in `$XDG_CONFIG_HOME` when they should be stored in `$XDG_DATA_HOME`. With the XDG specification I would expect to see in:

```
~/.config/composer/
composer.json
(maybe composer.lock)

~/.local/share/composer/
vendor/
somepackagedir/
```

or all of the above in `~/.local/share/composer`

I do have all 3 of XDG_CONFIG_HOME, XDG_DATA_HOME, and XDG_CACHE_HOME set.

I'm using

``` sh
$ composer --version
Composer version 1.0-dev (8bf46768f3f5397c259b77321b2e887a53624adb) 2016-03-10 16:31:09
```