Fix tests warning in gun_test.exs

Fixes the following warnings:

warning: variable "response" is unused (if the variable is not meant to be used, prefix it with an underscore) test/tesla/adapter/gun_test.exs:51: Tesla.Adapter.GunTest."test url without path, but with query"/1

warning: variable "response" is unused (if the variable is not meant to be used, prefix it with an underscore)

	  test/tesla/adapter/gun_test.exs:107: Tesla.Adapter.GunTest."test certificates_verification option"/1