TargetPath not set correctly for contentFiles

When trying to use contentFiles with UWP project, the files were not copied to the expected subdirectory.

Here's the nuspec:
https://gist.github.com/clairernovotny/5d007fd323a8393412b9

I expected Doc2.txt to be in the `AppX\Resources\Doc2.txt` directory. Instead, it was in `AppX\Doc2.txt`

Looking at a diagnostic build, it seems like the TargetPath metadata was not set correctly:

```
1> C:\Users\oren\.nuget\packages\TestWhitelist\1.2.0\contentFiles\any\uap10.0\Resources\Doc2.txt
1> NuGetItemType=Content
1> NuGetPackageId=TestWhitelist
1> NuGetPackageVersion=1.2.0
1> OriginalItemSpec=C:\Users\oren\.nuget\packages\TestWhitelist\1.2.0\contentFiles\any\uap10.0\Resources\Doc2.txt
1> TargetPath=Doc2.txt
```

cc @clairernovotny @jasonmalinowski

Moved from: https://github.com/NuGet/Home/issues/2001