network/omaha: fix go vet complaints
network/omaha: fix go vet complaints

network/omaha/protocol_test.go:83: ExampleOmaha_NewResponse refers to unknown identifier: Omaha
network/omaha/protocol_test.go:134: ExampleOmaha_NewRequest refers to unknown identifier: Omaha

marineam/mantle,mjg59/mantle,dm0-/mantle,glevand/coreos--mantle,mischief/mantle,quantum/coreos-mantle,quantum/coreos-mantle,dm0-/mantle,marineam/mantle,mischief/mantle,glevand/coreos--mantle,mjg59/mantle