Assembly reference specific version matches visual studio behaviour

Scenario
-----------
Updating existing project files to change assembly reference hint paths, and want as little merge conflicts as possible

Problem
----------
`SpecificVersion` boolean is serialized `ToLower()` which is not the Visual Studio behaviour

Fubu
```xml
 <Reference Include="log4net">
      <SpecificVersion>false</SpecificVersion>
      <HintPath>Lib\log4netdll</HintPath>
    </Reference>
```

Visual Studio
```xml
 <Reference Include="log4net">
      <SpecificVersion>False</SpecificVersion>
      <HintPath>Lib\log4netdll</HintPath>
    </Reference>
```

Note the difference in boolean casing.


Fix
----
This pull request removes the `ToLower` in `AssemblyReference` adds a unit test for the desired behaviour `AssemblyReferenceTester`