emptyStringIsFalse flag now works with any Object.

Previously, only String types were checked for being empty. Now
the Formatter is used to format the Object before checking for
being empty.

src/main/java/com/samskivert/mustache/Mustache.java

@@ -203,7 +203,7 @@ public class Mustache {
           * then empty strings are considered falsey. If {@link #zeroIsFalse} is true, then zero
           * values are considered falsey. */
         public boolean isFalsey (Object value) {
-            return (emptyStringIsFalse && "".equals(value)) ||
+            return (emptyStringIsFalse && "".equals(formatter.format(value))) ||
             (zeroIsFalse && (value instanceof Number) && ((Number)value).longValue() == 0);
         }