Exclude non-proxyable interface methods

Two changes need to be made to `InterfaceMembersCollector`:

 1. `AcceptMethod` needs to be called with `onlyVirtuals: true`, so that
    it will filter out non-overridable methods during pre-screening.

 2. For such filtered-out methods, it needs to return `null` in order to
    avoid `MinimalisticMethodGenerator` (which would mark the method it
    creates as an override).

These changes have been copied from `ClassMembersCollector`. Note that
the two classes are now almost identical.

src/Castle.Core/DynamicProxy/Contributors/InterfaceMembersCollector.cs

@@ -33,7 +33,13 @@ protected override MetaMethod GetMethodToGenerate(MethodInfo method, IProxyGener
 				return null;
 			}
 
-			var proxyable = AcceptMethod(method, false, hook);
+			var proxyable = AcceptMethod(method, true, hook);
+			if (!proxyable && !method.IsAbstract)
+			{
+				// we don't need to do anything
+				return null;
+			}
+
 			return new MetaMethod(method, method, isStandalone, proxyable, false);
 		}
 	}