small fix in filter tests

lib/web3/filter.js

@@ -72,7 +72,7 @@ var getOptions = function (options) {
     }
 
     var asBlockNumber = function (n) {
-        if (n === null) {
+        if (n === null || typeof n === 'undefined') {
             return null;
         } else if (n === 'latest' || n === 'pending') {
            return n;