NERDTreeHijackNetrw ignored when using Default Key: e

Docs state:

```
Default key: e
Map option: NERDTreeMapOpenExpl
Applies to: files and directories.

|:edit|s the selected directory, or the selected file's directory. This could
result in a NERD tree or a netrw being opened, depending on
|'NERDTreeHijackNetrw'|.
```

But even with NERDTreeHijackNetrw set to 0, pressing 'e' on a directory still opens the directory in a 'secondary' NERDTree.
