[Semantic] Handled default parameter value conversion in default

unresolved parameter.

ICSharpCode.NRefactory/Semantics/ConversionResolveResult.cs

@@ -64,12 +64,5 @@ namespace ICSharpCode.NRefactory.Semantics
 			return new [] { Input };
 		}
 
-		public override object ConstantValue {
-			get { 
-				if (Conversion.IsNullableConversion)
-					return Input.ConstantValue;
-				return null;
-			}
-		}
 	}
 }

ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultUnresolvedParameter.cs

@@ -235,6 +235,11 @@ namespace ICSharpCode.NRefactory.TypeSystem.Implementation
 			public object ConstantValue {
 				get {
 					ResolveResult rr = LazyInit.VolatileRead(ref this.resolvedDefaultValue);
+					if (rr is ConversionResolveResult) {
+						var crr = (ConversionResolveResult)rr;
+						if (crr.Conversion.IsNullableConversion)
+							return crr.Input.ConstantValue;
+					}
 					if (rr != null) {
 						return rr.ConstantValue;
 					} else {