wrong type check of adapter

[VantagePOS]

In core_enforcer.dart, the type of the adapter is checked using adapter.runtimeType == FilteredAdapter, which would expect an exact match of FilteredAdapter, however FilteredAdapter is an abstract class so this will never evaluate to true.

This should be changed to: adapter is FilteredAdapter

Needs to be changed in two places:

dart-casbin/lib/src/core_enforcer.dart

Line 179 in 9c72f3f

if (adapter.runtimeType == FilteredAdapter) {

dart-casbin/lib/src/core_enforcer.dart

Line 198 in 9c72f3f

if (adapter.runtimeType == FilteredAdapter) {

[hsluoyz]

@VantagePOS can you make a PR to fix it?