Address issue #873 (#874)

* Address issue #873

* Be explicit as to what we support writing

* Use normal test infrastructure

* Restore writing primitives

* Restore style

* Can't verify .net core assembly

Co-authored-by: Jb Evain <jb@evain.net>

Mono.Cecil/AssemblyWriter.cs

@@ -2192,6 +2192,7 @@ SignatureWriter GetConstantSignature (ElementType type, object value)
 			case ElementType.None:
 			case ElementType.Var:
 			case ElementType.MVar:
+			case ElementType.GenericInst:
 				signature.WriteInt32 (0);
 				break;
 			case ElementType.String:

Test/Mono.Cecil.Tests/PortablePdbTests.cs

@@ -453,6 +453,23 @@ public void NullClassConstant ()
 			}, symbolReaderProvider: typeof (PortablePdbReaderProvider), symbolWriterProvider: typeof (PortablePdbWriterProvider));
 		}
 
+		[Test]
+		public void NullGenericInstConstant ()
+		{
+			TestModule ("NullConst.dll", module => {
+				var type = module.GetType ("NullConst.Program");
+				var method = type.GetMethod ("MakeConst");
+				var symbol = method.DebugInformation;
+
+				Assert.IsNotNull (symbol);
+				Assert.AreEqual (1, symbol.Scope.Constants.Count);
+
+				var a = symbol.Scope.Constants [0];
+				Assert.AreEqual ("thing", a.Name);
+				Assert.AreEqual (null, a.Value);
+			}, verify: false, symbolReaderProvider: typeof (PortablePdbReaderProvider), symbolWriterProvider: typeof (PortablePdbWriterProvider));
+		}
+
 		[Test]
 		public void InvalidConstantRecord ()
 		{