fix #337: Code inspection: "convert to constant"

Author: siegfriedpammer

src/Libraries/NRefactory/ICSharpCode.NRefactory.CSharp.Refactoring/CodeIssues/Synced/PracticesAndImprovements/ConvertToConstantIssue.cs

@@ -180,7 +180,7 @@ public override void VisitVariableDeclarationStatement (VariableDeclarationState
 					return;
 
 				var returnTypeRR = ctx.Resolve(varDecl.Type);
-				if (returnTypeRR.Type.IsReferenceType.HasValue && returnTypeRR.Type.IsReferenceType.Value)
+				if (!IsValidConstType(returnTypeRR.Type))
 					return;
 
 				var variable = varDecl.Variables.First();