BibTeX Plugin: Force fieldnames to be lowercase

Fixes #5591

plugins/tiddlywiki/bibtex/deserializer.js

@@ -39,7 +39,7 @@ exports["application/x-bibtex"] = function(text,fields) {
 			"bibtex-entry-type": entry.entryType
 		};
 		$tw.utils.each(entry.entryTags,function(value,name) {
-			fields["bibtex-" + name] = value;
+			fields["bibtex-" + name.toLowerCase()] = value;
 		});
 		results.push(fields);
 	});