Fix EZP-21358: ezjscAjaxContent: Image's exif fields MakerNote and UserComment must be base64 encoded

ezjscAjaxContent now returns image's exif fields MakerNote and UserComment base64 encoded.
Merge pull request #707 from ezsystems/ezp-21358-exif_base64

Fix EZP-21358: ezjscAjaxContent: Image's exif fields MakerNote and UserComment must be base64 encoded