Accept prefixed xml attributes with null value

This changes makes PrefixedAttribute work the same way as
UnprefixedAttribute with respect to null values:

<t p:a={ null: String }/> is accepted and results in <t/>

Author: khernyo

src/library/scala/xml/PrefixedAttribute.scala

@@ -13,22 +13,25 @@ package scala.xml
  *
  *  @param pre   ...
  *  @param key   ...
- *  @param value the attribute value, which may not be null
+ *  @param value the attribute value
  *  @param next  ...
  */
 class PrefixedAttribute(
   val pre: String,
   val key: String,
   val value: Seq[Node],
-  val next: MetaData)
+  val next1: MetaData)
 extends Attribute
 {
-  if (value eq null)
-    throw new UnsupportedOperationException("value is null")
+  val next = if (value ne null) next1 else next1.remove(key)
 
-  /** same as this(key, Utility.parseAttributeValue(value), next) */
+  /** same as this(pre, key, Text(value), next), or no attribute if value is null */
   def this(pre: String, key: String, value: String, next: MetaData) =
-    this(pre, key, Text(value), next)
+    this(pre, key, if (value ne null) Text(value) else null: NodeSeq, next)
+
+  /** same as this(pre, key, value.get, next), or no attribute if value is None */
+  def this(pre: String, key: String, value: Option[Seq[Node]], next: MetaData) =
+    this(pre, key, value.orNull, next)
 
   /** Returns a copy of this unprefixed attribute with the given
    *  next field.

test/files/run/xml-attribute.scala

@@ -5,10 +5,29 @@ object Test {
     val noAttr = <t/>
     val attrNull = <t a={ null: String }/>
     val attrNone = <t a={ None: Option[Seq[Node]] }/>
+    val preAttrNull = <t p:a={ null: String }/>
+    val preAttrNone = <t p:a={ None: Option[Seq[Node]] }/>
     assert(noAttr == attrNull)
     assert(noAttr == attrNone)
-    assert(noAttr.toString() == "<t></t>")
-    assert(attrNull.toString() == "<t></t>")
-    assert(attrNone.toString() == "<t></t>")
+    assert(noAttr == preAttrNull)
+    assert(noAttr == preAttrNone)
+
+    val noAttrStr = "<t></t>"
+    assert(noAttr.toString() == noAttrStr)
+    assert(attrNull.toString() == noAttrStr)
+    assert(attrNone.toString() == noAttrStr)
+    assert(preAttrNull.toString() == noAttrStr)
+    assert(preAttrNone.toString() == noAttrStr)
+
+    val xml1 = <t b="1" d="2"/>
+    val xml2 = <t a={ null: String } p:a={ null: String } b="1" c={ null: String } d="2"/>
+    val xml3 = <t b="1" c={ null: String } d="2" a={ null: String } p:a={ null: String }/>
+    assert(xml1 == xml2)
+    assert(xml1 == xml3)
+
+    val xml1Str = "<t d=\"2\" b=\"1\"></t>"
+    assert(xml1.toString() == xml1Str)
+    assert(xml2.toString() == xml1Str)
+    assert(xml3.toString() == xml1Str)
   }
 }