Fix IsSuccessful on RestResponse<T> (#1676)

Closes #1675

Author: kingjan1999

src/RestSharp/Response/RestResponse.cs

@@ -42,6 +42,7 @@ public static RestResponse<T> FromResponse(RestResponse response)
             ErrorMessage      = response.ErrorMessage,
             ErrorException    = response.ErrorException,
             Headers           = response.Headers,
+            IsSuccessful      = response.IsSuccessful,
             ResponseStatus    = response.ResponseStatus,
             ResponseUri       = response.ResponseUri,
             Server            = response.Server,

test/RestSharp.IntegrationTests/StatusCodeTests.cs

@@ -36,6 +36,7 @@ public async Task ContentType_Additional_Information() {
         var response = await _client.ExecuteAsync<TestResponse>(request);
 
         response.StatusCode.Should().Be(HttpStatusCode.OK);
+        response.IsSuccessful.Should().BeTrue();
     }
 
     [Fact]