Fix create_metadata output
Fix create_metadata output

The upgrade to `xmlbuilder2` broke some generated XMLs, the metadata is part of them as of [AuthNRequest](https://github.com/Clever/saml2/pull/227#issuecomment-772937111), there is probably more if the XML is build with arrays.

Explanation, given this code:
```
root.ele({
  number: [ 
    "one", 
    "two",
    { three: { "@val": 3 } }
  ]
});
```

`xmlbuilder` used to [render array like that](https://github.com/oozcitak/xmlbuilder-js/wiki/Conversion-From-Object#arrays):
```
<number>
  <one/>
  <two/>
  <three val="3"/>
</number>
```

And  `xmlbuilder2` now [renders them like that](https://oozcitak.github.io/xmlbuilder2/object-conversion.html):
```
<number>one</number>
<number>two</number>
<number>
  <three val="3"/>
</number>
```

There was the flag `separateArrayItems` in `xmlbuilder` to change the behavior but I do not see it in `xmlbuilder2`.
Clever/saml2