Prebid.js: SizeConfig shouldn't merge sizes of different labels together

Type of issue

This is a bug.

Description

When resolving the bid/adUnit sizes, Prebid doesn’t account for size configs with the same media query but different labels.

Steps to reproduce

With a size config:

// Notice that the media queries are the same
[
  {
    mediaQuery: "(min-width: 768px) and (max-width: 1199px)",
    sizesSupported: [[970, 90], [728, 90], [300, 250]],
    labels: ["myLabelA"]
  },
  {
    mediaQuery: "(min-width: 768px) and (max-width: 1199px)",
    sizesSupported: [[728, 90], [300, 250]],
    labels: ["myLabelB"]
  }
]

and a bid with { labelAny: ['myLabelA'] }, Prebid sends all the five sizes to DFP. I expected it to send only myLabelA sizes to DFP: [[970, 90], [728, 90], [300, 250]].

My use case

I have a bunch of different sections on the website (header, footer, sidebar, fixed ads, in-content, etc) like so:

{
  header: {
    xs: {
      mediaQuery: '(min-width: 576px) and (max-width: 767px)',
      sizes: [[468, 60], [600, 300]]
    },
    sm: {
      mediaQuery: '(max-width: 575px)',
      sizes: [[468, 60], [300, 250]]
    }
  ],
  sidebar: [...]
}

And then in the bids:

// MyAdUnitA
[
  {
    bidder: 'sovrn',
    params: { tagid: '123456' },
    labelAny: ['header-xs', 'header-sm'] },
  },
  // ...
]