Assembly reference private matches visual studio behaviour

[awelburn]

Variation of #83 but with the private element instead
Text copied from #83 for reference:

Scenario

Updating existing project files to change assembly reference hint paths, and want as little merge conflicts as possible

Problem

Private boolean is serialized ToLower() which is not the Visual Studio behaviour

Fubu

<Reference Include="log4net">
    <HintPath>Lib\log4netdll</HintPath>
    <Private>false</Private>
</Reference>

Visual Studio

<Reference Include="log4net">
    <HintPath>Lib\log4netdll</HintPath>
    <Private>False</Private>
</Reference>

Note the difference in boolean casing.