Made INVALID_USER_PERSON_ID a constant. (#63)

Author: Kevin Hellemun

BunqSdk.Tests/Http/ResponseIdOnBadRequestTest.cs

@@ -16,7 +16,7 @@ public class ResponseIdOnBadRequestTest : BunqSdkTestBase
         /// <summary>
         /// Invalid user id to trigger BadRequestException
         /// </summary>
-        private static int INVALID_USER_PERSON_ID = 0;
+        private const int INVALID_USER_PERSON_ID = 0;
 
         [Fact]
         public void TestBadRequestWithResponseId()