Preserve whitespace for metadata as attributes

[cdmihai]

Input:

<Project xmlns='http://schemas.microsoft.com/developer/msbuild/2003'>
    <ItemGroup>
        <i1 Include='i'
            a='v'
            b='v' />
    </ItemGroup>
</Project>

Action:

itemProjectElement.AddMetadata("m", "v", true);

Output:

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
    <ItemGroup>
        <i1 Include="i" a="v" b="v" m="v" />
    </ItemGroup>
</Project>