XmlDsigXPathTransform sample fails

Moved from https://github.com/aspnet/Docs/issues/5641
cc @rpetrusha
See [XmlDsigXPathTransform Class
](https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.xml.xmldsigxpathtransform?view=netframework-4.7)

test sample. Copy from [this bug](https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems/edit/200269)

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Problem Description
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Description: After MS16-035 security update this code from MSDN doesn't work:

`[Edit: here is the dotnet link]` See [XmlDsigXPathTransform Class
](https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.xml.xmldsigxpathtransform?view=netframework-4.7)

https://msdn.microsoft.com/library/system.security.cryptography.xml.xmldsigxpathtransform.xmldsigxpathtransform%28v=vs.110%29.aspx

`[Edit - not needed to repo]`
Repro Steps:- Install MS16-035
- compile and execute exampkle from: https://msdn.microsoft.com/it-it/library/system.security.cryptography.xml.xmldsigxpathtransform.xmldsigxpathtransform%28v=vs.110%29.aspx

Impact:Unable to verify XML signed document

---------------------------

Run sample and you get the following output:

```
New XML file created.
XML file signed.
Verifying signature...
The XML signature is not valid.
```