Fix tests warning in follow_redirects_test.exs
Fixes the following warnings:
warning: variable "env" is unused (if the variable is not meant to be used, prefix it with an underscore)
test/tesla/middleware/follow_redirects_test.exs:223: Tesla.Middleware.FollowRedirectsTest."test headers Keep authorization header on redirect to the same domain"/1
warning: variable "env" is unused (if the variable is not meant to be used, prefix it with an underscore)
test/tesla/middleware/follow_redirects_test.exs:245: Tesla.Middleware.FollowRedirectsTest."test headers Strip authorization header on redirect to a different domain"/1
warning: variable "env" is unused (if the variable is not meant to be used, prefix it with an underscore)
test/tesla/middleware/follow_redirects_test.exs:266: Tesla.Middleware.FollowRedirectsTest."test headers Keep custom host header on redirect to a different domain"/1
warning: variable "env" is unused (if the variable is not meant to be used, prefix it with an underscore)
test/tesla/middleware/follow_redirects_test.exs:285: Tesla.Middleware.FollowRedirectsTest."test headers Strip custom host header on redirect to a different domain"/1