使用范型兼容同一个请求返回不同数据类型

[mrcwl]

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Issue Info

Info	Value
Platform Name	ios / android
Platform Version	e.g. 1.5.0 / 12.0 / 9.0
Dio Version	2.1.13
Android Studio / Xcode Version	e.g. Android Studio 3.3.2 / Xcode 10.2.1
Repro rate	e.g. all the time (100%) / sometimes x% / only once
Repro with our demo prj	e.g. does it happen with our demo project?
Demo project link	e.g. link to a demo project that highlights the issue

Issue Description and Steps

我的接口200时返回bool类型，但是400的时候返回的是一个json对象，我加入范型post
接口返回400的时候提示该错误：
flutter: #0 Dio._assureResponse (package:dio/src/dio.dart:1012:9)
#1 Dio.resolve. (package:dio/src/dio.dart:309:14)
#2 _rootRunUnary (dart:async/zone.dart:1132:38)
#3 _CustomZone.runUnary (dart:async/zone.dart:1029:19)
#4 _FutureListener.handleValue (dart:async/future_impl.dart:126:18)
#5 Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:639:45)
#6 Future._propagateToListeners (dart:async/future_impl.dart:668:32)
#7 Future._completeWithValue (dart:async/future_impl.dart:483:5)
#8 Future._asyncComplete. (dart:async/future_impl.dart:513:7)
#9 _rootRun (dart:async/zone.dart:1124:13)
#10 _CustomZone.run (dart:async/zone.dart:1021:19)
#11 _CustomZone.runGuarded (dart:async/zone.dart:923:7)
#12 _CustomZone.bindCallbackGuarded. (dart:async/zone.dart:963:23)
#13 _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
#14 _startMicrotaskLoop (dart:a<…>

[mrcwl]

似乎是responseres类型转换时出现问题

[wendux]

返回类型是动态的情况可以不指定类型参数，此时T为dynamic