Improve xml.Attribute.copy and xml.MetaData.iterator

[scabug]

= problem =
When transforming XML, it is often necessary to transform attribute values as well as elements. It would be nice to be able to write something like:

case elem: Elem => elem.copy(attributes=
  for (attr <- elem.attributes) yield attr match {
    case attr@Attribute("attrToChange", _, _) => attr.copy(value=Text("newValue"))
    case other => other
  }
)

= analysis =

This doesn't work in Scala 2.8.1 because xml.Attribute.copy only allows the next attribute to be changed, and because map over an xml.MetaData yields an Iterable[MetaData] not a MetaData.

= enhancement recommendation =

To solve the copying problem, xml.Attribute could grow a method like one of the xml.Attribute.apply methods, e.g.:

def copy(pre: String = this.pre, key: String = this.key, value: Seq[Node] = this.value, next: MetaData = this.next): Attribute

Solving the map problem probably means updating Attribute to use the new collections APIs better - e.g. it should probably implement IterableLike[MetaData, MetaData] which I think means it needs a simple Builder that sets the next attributes appropriately.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-4050?orig=1
Reporter: Steven Bethard (bethard)

[scabug]

@SethTisue said:
The scala-xml library is now community-maintained. Issues with it are now tracked at https://github.com/scala/scala-xml/issues instead of here in the Scala JIRA.

Interested community members: if you consider this issue significant, feel free to open a new issue for it on GitHub, with links in both directions.