Core - Corrected disposable pattern usage

Core - Further nullable refactoring..

src/Artemis.Core/Artemis.Core.csproj

@@ -13,7 +13,8 @@
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">
     <PlatformTarget>x64</PlatformTarget>
     <DocumentationFile>bin\x64\Debug\Artemis.Core.xml</DocumentationFile>
-    <NoWarn>1701;1702</NoWarn>
+    <NoWarn></NoWarn>
+    <WarningLevel>5</WarningLevel>
   </PropertyGroup>
 
   <PropertyGroup>
@@ -26,6 +27,7 @@
     <NrtRequiredVcs>git</NrtRequiredVcs>
     <NrtShowRevision>true</NrtShowRevision>
     <Nullable>enable</Nullable>
+    <AnalysisLevel>latest</AnalysisLevel>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'">