Update currency array warning

[bretg]

PBS currently converts incoming currency arrays to a containing the first element of the array. So bid adapters only see this first element.

We discussed in committee the ability to preserve the incoming array and just use the first element as the currency for bid response conversion.

However, the ORTB response structure doesn't support bids in multiple currencies -- cur is at the very top level - the same level as seatbid. So if some bidders were to bid in USD and others in CAD, there's no standard place to put that info. seatbid.bid has a price but not a cur. Of course Prebid could add an extension, but that wouldn't be standard.
We could add a non-default Prebid-specific behavior where a request parameter says "let bidders bid in any of the currencies in the cur array", which indicates that the client is ready to receive seatbid.bid.ext.cur as the bid currency. I don't see this becoming the default in PBS because AMP, Mobile, etc.

At this point, we don't view this as high enough priority to define or implement.

Instead, the suggestion is that we update the warning. Currently the message is: Cur parameter contains more than one currency. CAD will be used

The proposal is to update it to: a single currency (CAD) has been chosen for the request. ORTB 2.6 requires that all responses are in the same currency.

[patmmccann]

This wording works well and instead of reading like a concern, reads as informational