fix invalid json-ld image object alt property

JSON-LD ImageObject alt property is not recongized as a valid property. Therefore, logically replace the alt property with description without breaking other metadata and then delete it.

lib/jekyll-seo-tag/json_ld_drop.rb

@@ -59,6 +59,15 @@ module Jekyll
         hash = page_drop.image.to_h
         hash["url"]   = hash.delete("path")
         hash["@type"] = "imageObject"
+
+        if hash["alt"] && !hash["description"]
+          hash["description"] = hash["alt"]
+        end
+
+        if hash["alt"]
+          hash.delete("alt")
+        end
+
         hash
       end