Fix ruby 2.4 warnings in tests
Fix ruby 2.4 warnings in tests

```
warning: constant ::Fixnum is deprecated
warning: constant ::Bignum is deprecated
```

And simplify the test at the same time.

wspurgin/activeadmin,SoftSwiss/active_admin,vytenis-s/activeadmin,wspurgin/activeadmin,javierjulio/activeadmin,cmunozgar/activeadmin,mauriciopasquier/active_admin,waymondo/active_admin,vytenis-s/activeadmin,activeadmin/activeadmin,gogovan/activeadmin,mrjman/activeadmin,vytenis-s/activeadmin,mrjman/activeadmin,deivid-rodriguez/activeadmin,varyonic/activeadmin,wspurgin/activeadmin,maysam/activeadmin,vraravam/activeadmin,varyonic/activeadmin,mauriciopasquier/active_admin,mauriciopasquier/active_admin,cmunozgar/activeadmin,deivid-rodriguez/activeadmin,waymondo/active_admin,vraravam/activeadmin,SoftSwiss/active_admin,deivid-rodriguez/activeadmin,gogovan/activeadmin,vraravam/activeadmin,cmunozgar/activeadmin,gogovan/activeadmin,waymondo/active_admin,SoftSwiss/active_admin,javierjulio/activeadmin,siutin/activeadmin,siutin/activeadmin,mrjman/activeadmin,siutin/activeadmin,varyonic/activeadmin,maysam/activeadmin,maysam/activeadmin,javierjulio/activeadmin,activeadmin/activeadmin,activeadmin/activeadmin