Fix the issue that edit command allows duplicate emails or phone

[Elinengu]

Closes #154

The reason why the original if else statement does not work is that, as the the isSamePerson check if there is the same email or same phone number, hence in this statement:

        if (!personToEdit.isSamePerson(editedPerson) && model.hasPerson(editedPerson)) {
            throw new CommandException(MESSAGE_DUPLICATE_PERSON);
        }

The duplicate email/phone number can by pass the hasPerson check if the original phone number or email remains unchanged.