prettify xml NodeSeq and NodeBuffer

Before:

```
  scala> val ab = <a/><b/>
  ab: scala.xml.NodeBuffer = ArrayBuffer(<a/>, <b/>)

  scala> Prettifier.default(ab) 
  res0: String = ArrayBuffer(<a/>, <b/>)

  scala> val ab: NodeSeq = <a/><b/>
  ab: scala.xml.NodeSeq = NodeSeq(<a/>, <b/>)

  scala> Prettifier.default(ab) 
  res1: String = <a/><b/>(<a/>, <b/>)

```
After:

```
  scala> val ab = <a/><b/>
  ab: scala.xml.NodeBuffer = ArrayBuffer(<a/>, <b/>)

  scala> Prettifier.default(ab) 
  res0: String = <a/><b/>

  scala> val ab: NodeSeq = <a/><b/>
  ab: scala.xml.NodeSeq = NodeSeq(<a/>, <b/>)

  scala> Prettifier.default(ab) 
  res1: String = <a/><b/>
```