Prevent overlapping shipping rule range for quantity discount
Prevent overlapping shipping rule range for quantity discount

closes #743

OpenBazaar/openbazaar-go,hoffmabc/openbazaar-go,hoffmabc/openbazaar-go,gubatron/openbazaar-go,hoffmabc/openbazaar-go,OpenBazaar/openbazaar-go,OpenBazaar/openbazaar-go,gubatron/openbazaar-go,gubatron/openbazaar-go