add missing test file to gemspec
add missing test file to gemspec

Fixes #273 [ci skip]

lostisland/faraday,lostisland/faraday,jhass/faraday,daviswahl/faraday,jrafanie/faraday,jrafanie/faraday,zanker/faraday,flower-pot/faraday,ekampp/faraday,pvdb/faraday,luvtechno/faraday,iamjarvo/faraday,petems/faraday,hstrowd/faraday,petems/faraday,zanker/faraday,spyofchina/faraday,customink/faraday,pvdb/faraday,grosser/faraday,hrysd/faraday,spyofchina/faraday,zhaoxl/faraday,ngpestelos/faraday,O-I/faraday,O-I/faraday,mehulkar/faraday,ru-faraon/Faraday-is-an-HTTP-client,hrysd/faraday,engineyard/faraday,nevans/faraday,davidcornu/faraday,prcongithub/faraday,engineyard/faraday,ekampp/faraday,iamjarvo/faraday,prcongithub/faraday,jhass/faraday,davoclavo/faraday,davidcornu/faraday,apepper/faraday,santiagodoldan/faraday,mehulkar/faraday,zhaoxl/faraday,ru-faraon/Faraday-is-an-HTTP-client,customink/faraday,coinbase/faraday,coinbase/faraday,nevans/faraday,flower-pot/faraday,mauricionr/faraday,luvtechno/faraday,ngpestelos/faraday,mauricionr/faraday,hstrowd/faraday,stayhero/faraday,apepper/faraday,daviswahl/faraday,AvaelKross/faraday,AvaelKross/faraday,santiagodoldan/faraday,stayhero/faraday,davoclavo/faraday,grosser/faraday