Fix tests, address commentary from @braydonf.

lib/networks.js

@@ -75,7 +75,7 @@ function addNetwork(data) {
   });
 
   _.each(_.values(network), function(value) {
-    if (value && !_.isObject(value)) {
+    if (!_.isUndefined(value) && !_.isObject(value)) {
       networkMaps[value] = network;
     }
   });

test/networks.js

@@ -57,7 +57,7 @@ describe('Networks', function() {
     };
     networks.add(custom);
     var network = networks.get(undefined);
-    should.not.exist( network );
+    should.not.exist(network);
   });
 
   var constants = ['name', 'alias', 'pubkeyhash', 'scripthash', 'xpubkey', 'xprivkey'];