beforeXHR setRequestHeader not setting correct key/value


I have a search configured as follows:

                    minCharacters: 3,
                    localSearch: false,
                    apiSettings: {
                        url: '//{query}?entityFormat=png%3A150_150&contentType=JSON',
                        beforeXHR: function (xhr) {
                            xhr.setRequestHeader('Authorization', 'Basic XXXXXXXXX');
                        onResponse: function (mssResponse) {
                            var response = {
                                    results: []
                            $.each(mssResponse.resultDataList, function (index, item) {
                                var maxResults = 8;
                                if (index >= maxResults) {
                                    return false;
                                    title: item.returnedEntityId,
                                    description: item.source,
                                    image: item.content
                            return response;

However, in my web debugging proxy the header is set as follows:

Access-Control-Request-Headers: authorization

Manually updating the header to

Authorization: Basic XXXXXXXXX

and resending works.

This is perplexing, especially since I’m following the example just like Any insight?