Fixing issue where the ajaxHeaders are not being set for image requests

[maxshuty]

This PR fixes an issue where image requests with custom ajaxHeaders set do not actually use those ajaxHeaders. For example if I have this code below where I am dynamically adding images and each image has it's own ajaxHeaders set:

    urls.forEach((url, index) => {
        viewer.addTiledImage({
            tileSource: url,
            loadTilesWithAjax: true,
            width: 10,
            ajaxHeaders: {
                Authorization: tokens[index],
            },
        });
    });

The requests to not actually use the ajaxHeaders for these images until actual tile requests are made.

This PR fixes this issue.

cc: @iangilman as we chatted on Gitter about this.

[iangilman]

Beautiful, thank you!