Rebased and cleaned up #6279

## Issue summary
In `Zend\Config\Reader\Xml`, loading an XML file with elements having both attributes and a node value the node value is lost:

Loading this XML file:
```
<?xml version="1.0"?>
<config>
    <one foo="bar">bazbat</one>
</config>
```

Would result in an array like this:

```
array(1) {
  'one' =>
  array(2) {
    [0] =>
    array(2) {
      'foo' =>
      string(3) "bar"
    }
  }
}
```

## Fix

If an element has attributes then read the node value and place it in the special array key `_`

## Ack
Fix contributed by @catalint