Fixed <import>'s "optional" attribute.

Reported by:  Nick Gianakas


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@276304 13f79535-47bb-0310-9956-ffa450edef68

WHATSNEW

@@ -110,6 +110,9 @@ Fixed bugs:
   the -C switch to gcj.  This means you can now compile to native code
   with gcj which has been impossible in Ant < 1.6.2.
 
+* <import optional="false"> and <import optional="true">
+  behaved identically.
+
 Other changes:
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