Add ref to System.Memory

Author: mythz

tests/ServiceStack.Redis.Tests/ServiceStack.Redis.Tests.csproj

@@ -22,6 +22,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.5.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="3.9.0" />
     <PackageReference Include="NUnit" Version="3.9.0" />
+    <PackageReference Include="System.Memory" Version="4.5.1" />
   </ItemGroup>
 
   <PropertyGroup Condition=" '$(TargetFramework)' == 'net46' ">