Fix wrong type checking when using instanceName in combination with generic type parameter

[knaeckeKami]

Currently, this code snippet fails:

import 'package:get_it/get_it.dart';

void main(){
  GetIt.I.registerSingleton<String>("test", instanceName: "instanceName");

  final myString = GetIt.I<String>("instanceName");

  print(myString);
}

with the message:

Unhandled exception:
Invalid argument(s): Object with name instanceName has a different type (String) than the one that is inferred (String) where you call it
#0      GetIt.get (package:get_it/get_it.dart:80:11)
#1      GetIt.call (package:get_it/get_it.dart:98:12)
#2      main (package:neocortex_mobile/get_it_test.dart:9:27)
#3      _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:305:19)
#4      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:172:12)

The reason for this is, that the logic for checking the type or the object registered with instanceName is wrong:

 if (registeredObject.registrationType is! T) {
          print(T.toString());
          throw ArgumentError(
              "Object with name $instanceName has a different type (${registeredObject.registrationType.toString()}) than the one that is inferred (${T.toString()}) where you call it");
        }

registeredObject.registrationType is of Type Type and T is of Type String (in this case), so the comparison fails.
This only works if the caller does not use generic type parameters, because then T is of type dynamic, and dynamic is a bottom type, so the X is dynamic is always true.

This PR fixes this by checking the type of the actual instance.
I also added a test for this.