fields are case-insensitive;

--- a/src/Pure/Tools/bibtex.scala	Sat Oct 04 16:02:17 2014 +0200
+++ b/src/Pure/Tools/bibtex.scala	Sat Oct 04 16:11:39 2014 +0200
@@ -40,9 +40,9 @@
     optional_crossref: List[String],
     optional_other: List[String])
   {
-    def is_required(s: String): Boolean = required.contains(s)
+    def is_required(s: String): Boolean = required.contains(s.toLowerCase)
     def is_optional(s: String): Boolean =
-      optional_crossref.contains(s) || optional_other.contains(s)
+      optional_crossref.contains(s.toLowerCase) || optional_other.contains(s.toLowerCase)
 
     def fields: List[String] = required ::: optional_crossref ::: optional_other
     def template: String =