Convert to lower, not upper, for casecmp

This matches the CRuby code at this point, but it's not clear to
me how or why it was set up to use the upper case table many years
ago.

Fixes jruby#7946

core/src/main/java/org/jruby/util/StringSupport.java

@@ -2505,8 +2505,8 @@ public static int multiByteCasecmp(Encoding enc, ByteList value, ByteList otherV
 
             final int cl, ocl;
             if (Encoding.isAscii(c) && Encoding.isAscii(oc)) {
-                int dc = AsciiTables.ToUpperCaseTable[c];
-                int odc = AsciiTables.ToUpperCaseTable[oc];
+                int dc = AsciiTables.ToLowerCaseTable[c];
+                int odc = AsciiTables.ToLowerCaseTable[oc];
                 if (dc != odc) return dc < odc ? -1 : 1;
 
                 if (enc.isAsciiCompatible()) {