Support case insensitive bidder name in storedbidresponse

[onkarvhanumante]

PR makes changes to support case insensitive bidder names in ext.prebid.storedbidresponse.bidder

As shown below bidder name appnexus is known name to PBS. But APPNEXUS is unknown. Prior to PR changes PBS would have error out with unknown APPNEXUS bidder name messgae. PR has implemented insensitive bidder name comparison in ext.prebid.storedbidresponse.bidder

        "ext": {
          "appnexus": {
            "placementId": 12883451
          },
          "prebid": {
            "storedbidresponse": [
              {
                "bidder": "APPNEXUS",
                "id": "bidResponseId3"
              }
            ]
          }
        }

partially resolves #2400