mapper: Correct GetInterfaceType(Type,string)

In recent commits I had neglected to consider that some users of
dbus-sharp will be subclassing MarshalByRefObject.

This commit corrects that, adding a simple GetHierarchy function to
return the type and its ancestors.

src/ExportObject.cs

@@ -70,7 +70,7 @@ public MethodCaller GetPropertyAllCall (string iface)
 
 			if (null == calls.All) {
 				Type it = Mapper.GetInterfaceType (ObjectType, iface);
-				calls.All = null == it ? null : TypeImplementer.GenGetAllCall (it);
+				calls.All = TypeImplementer.GenGetAllCall (it);
 			}
 
 			return calls.All;

src/Mapper.cs

@@ -37,7 +37,22 @@ public static string GetArgumentName (ICustomAttributeProvider attrProvider, str
 
 		public static Type GetInterfaceType(Type type, string iface)
 		{
-			return type.GetInterfaces ().FirstOrDefault (x => iface == GetInterfaceName (x));
+			return type.GetInterfaces()
+				.Concat(GetHierarchy(type))
+				.FirstOrDefault (x => iface == GetInterfaceName (x));
+		}
+
+		private static IEnumerable<Type> GetHierarchy(Type type)
+		{
+			if (IsPublic (type)) {
+				yield return type;
+			}
+
+			foreach (var super in GetHierarchy (type.BaseType)) {
+				if (IsPublic (type)) {
+					yield return super;
+				}
+			}
 		}
 
 		public static IEnumerable<PropertyInfo> GetPublicProperties (Type type)