Nomad inproperly unpack tar.gz artifact archives with symlinks

### Nomad version
Nomad v0.5.4

We've made artifact with many symlinks in it. Is our case this links goes to so(shared objects), like in this sample:

```
lrwxrwxrwx 1 root root 18 Feb 8 15:40 libcrypto.so -> libcrypto.so.1.0.0
-r-xr-xr-x 1 root root 2308736 Feb 8 14:42 libcrypto.so.1.0.0
lrwxrwxrwx 1 root root 17 Feb 8 15:40 libcurses.so -> libncurses.so.5.9
-rwxrwxr-x 1 root root 142920 Feb 8 15:41 libncurses.so.5.9
lrwxrwxrwx 1 root root 21 Feb 8 15:40 libecpg_compat.so -> libecpg_compat.so.3.5
lrwxrwxrwx 1 root root 21 Feb 8 15:40 libecpg_compat.so.3 -> libecpg_compat.so.3.5
-rwxr-xr-x 1 root root 19024 Feb 8 14:42 libecpg_compat.so.3.5
```

Then in job file we've specifed follow atrifact declaration:

```
artifact
{
source = "http://docker.local/td-agent.3.tar.gz"
}
```

Artifact was successfully downloaded, and was extracted, but the contents of extracted dir, differs from
that which was in tar.gz archive:

```
-rwxrwxrwx 1 root root 0 Feb 8 15:41 libcrypto.so
-r-xr-xr-x 1 root root 2308736 Feb 8 15:41 libcrypto.so.1.0.0
-rwxrwxrwx 1 root root 0 Feb 8 15:41 libcurses.so
-rwxrwxr-x 1 root root 142920 Feb 8 15:41 libncurses.so.5.9
-rwxrwxrwx 1 root root 0 Feb 8 15:41 libecpg_compat.so
-rwxrwxrwx 1 root root 0 Feb 8 15:41 libecpg_compat.so.3
-rwxr-xr-x 1 root root 19024 Feb 8 15:41 libecpg_compat.so.3.5
```

As you can see, all symlinks after extract turn into simple files with zero length