support nested CDATA

[fommil]

according to https://en.wikipedia.org/wiki/CDATA#Nesting

A CDATA section cannot contain the string "]]>" and therefore it is not possible for a CDATA section to contain nested CDATA sections. The preferred approach to using CDATA sections for encoding text that contains the triad "]]>" is to use multiple CDATA sections by splitting each occurrence of the triad just before the ">". For example, to encode "]]>" one would write:
<![CDATA[]]]]><![CDATA[>]]>
This means that to encode "]]>" in the middle of a CDATA section, replace all occurrences of "]]>" with the following:
]]]]><![CDATA[>
This effectively stops and restarts the CDATA section.

It looks like we're not doing this in scala.xml, e.g.

      val top =
        """<?xml version="1.0" encoding="UTF-8"?>
          |<GRAND
          |    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="vast.xsd" version="2.0">
          |    <Foo><![CDATA[%s]]></Foo>
          |</GRAND>""".stripMargin

      println(PCData(top).toString)

gives

<![CDATA[<?xml version="1.0" encoding="UTF-8"?>
<GRAND
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="vast.xsd" version="2.0">
    <Foo><![CDATA[%s]]></Foo>
</GRAND>]]>

[fommil]

it looks like this could be supported by applying

.replaceAll("]]>", "]]]]><![CDATA[>")

in PCData.apply

Workaround is to manually remember to .safe after creating a PCData

  implicit class PCDataOps(orig: PCData) {
    def safe: PCData = PCData(orig.text.replaceAll("]]>", "]]]]><![CDATA[>"))
  }