TargetPath not set correctly for contentFiles

[nkolev92]

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: NuGet/Home#2001