XmlPoke task not updating an element value

**Repro:**
- Create a xml file called 'test.xml' with following content
```xml

Book

```
- Create a project file called 'test.proj' with following content
```xml

```
- Run `dotnet msbuild /t:Test`

**Expected**
```xml

Car

```

**Actual**
```xml

```

**Environment Info**
```
Microsoft (R) Build Engine version 15.3.234.47922 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.
```
**dotnet --info**
```
.NET Command Line Tools (2.0.0-preview2-006082)

Product Information:
Version: 2.0.0-preview2-006082
Commit SHA-1 hash: 6453cb4e15

Runtime Environment:
OS Name: Windows
OS Version: 10.0.15063
OS Platform: Windows
RID: win10-x64
Base Path: C:\Users\kichalla\.dotnet\x64\sdk\2.0.0-preview2-006082\

Microsoft .NET Core Shared Framework Host

Version : 2.0.0-preview2-25309-07
Build : 41f5fc94eedc889f086800c23f35bf14a8c75a9f
```