explicitly pass one argument to is_loaded()

Without the brackets, several Perl setups evidently interpreted this
differently:

http://cpantesters.org/cpan/report/da20f732-6c29-1014-80d0-99b2b77d427c
http://cpantesters.org/cpan/report/aad51258-fed7-11e2-8a37-e2882d12fa71
http://cpantesters.org/cpan/report/80bbfc0c-fefa-11e2-8a37-e2882d12fa71
http://cpantesters.org/cpan/report/c3b46b06-fec9-11e2-bbf2-f293a9e2ac29
http://cpantesters.org/cpan/report/c57e6572-fec9-11e2-bbf2-f293a9e2ac29
http://cpantesters.org/cpan/report/2c9fb592-fe81-11e2-9ed9-03bdf1ff63fb
http://cpantesters.org/cpan/report/c39c2ce2-fe80-11e2-86f4-e206f2ff63fb
http://cpantesters.org/cpan/report/2bce1906-fe81-11e2-9ed9-03bdf1ff63fb
v1.2.5

  - Fix test failures on some Perl setups.