Correcting 'ignores unknown attributes' test
Correcting 'ignores unknown attributes' test

While the previous test may run without error, it isn't asserting
anything interesting. By switching to check on `:@unknown` instance
variable, the assertion is inline with setting the `:unknown`
attribute during book initialization.

lotus/model,benlovell/model,vyper/model,fuadsaud/model,mymai91/model,hanami/model,brennovich/model,taylorfinnell/model,hlegius/model,habutre/model,pascalbetz/hanami-model,cllns/hanami-model,AlfonsoUceda/model,guilhermeFranco/model,theocodes/model,hanami/model,dsnipe/model