Fix test error
Fix test error

when running make test (go version go1.10.2 linux/amd64) :
client/graphite/read_test.go:113: Errorf format %s has arg nil of wrong type untyped nil
FAIL	github.com/criteo/graphite-remote-adapter/client/graphite [build failed]

Thib17/graphite-remote-adapter,Thib17/graphite-remote-adapter,Thib17/graphite-remote-adapter