This comes out of a request from Overdrive.

When we create a hold through the Overdrive API, it wants an email address to use for the hold notification. If we don't know a patron's email address, we use the library's value for the "Default email address to use when sending vendor hold notifications" setting. This is an address guaranteed to bounce or trash all incoming email.

Nobody's happy with this solution, so Overdrive has changed the holds endpoint, allowing it to take the argument "ignoreHoldEmail". If that is set to "true", we can omit the "emailAddress" argument, the hold will go through, and no hold notification will be sent.

We can't get rid of the "Default email address to use when sending vendor hold notifications" setting altogether, because some other vendor APIs need it.

I don't see a good isolated test of OverdriveAPI.place_hold, so we'll need to add more test coverage than we otherwise would for a small change like this.