XmlPeek with XPath functions

### What You Are Seeing?

XmlPeek is not working properly with XPath functions.

### What is Expected?

It works.

### What version of Cake are you using?

0.17

### Are you running on a 32 or 64 bit system?

64bit

### What environment are you running on? Windows? Linux? Mac?

Windows 10

### Are you running on a CI Server? If so, which one?

No.

### How Did You Get This To Happen? (Steps to Reproduce)

XmlPeek seems to be not happy with XPath functions. For example, querying string works as expected:

```c#
var packageType = XmlPeek(config, "/Config/Package/@type");
```

But not with functions like `string-length`:

```c#
var packageType = XmlPeek(config, "string-length(/Config/Package/@type)");
```

It throws ` System.Xml.XPath.XPathException` saying `Expression must evaluate to a node-set`.

Start at the XmlPeek entry point and reproduce the two XPath expressions from the issue on Cake 0.17 or the relevant current code. Compare the node-selection query with the string-length query and identify the existing handling for XPath results. Done means XPath functions no longer raise the reported node-set exception while the existing query still works.

Written by the indexing model from the issue text.