XmlPoke strips out DTD

### Issue Description
When modifying an XML file that contains DTD using the `XmlPoke` task the DTD is stripped out in the resulting XML file.

### Steps to Reproduce
The following sample writes an XML file with DTD to the disk. It serves as input for the `XmlPoke` task which modifies the content and writes a new XML file. Comparing the two XML files shows that the modified file no longer contains the DTD.

```xml

original.xml
modified.xml

<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "https://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist d1p1:version="" xmlns:d1p1="1.0">
<dict>
<key>archiveVersion</key>
<string>1</string>
</dict>
</plist>

```

### Expected Behavior
The file modified by the `XmlPoke` task contains the DTD just like before the modification.