fixed jsonrpc response 0 not handled properly

dist/ethereum.js

@@ -979,7 +979,7 @@ var isValidResponse = function (response) {
         !response.error &&
         response.jsonrpc === '2.0' &&
         typeof response.id === 'number' &&
-        (!!response.result || typeof response.result === 'boolean');
+        response.result !== undefined; // only undefined is not valid json object
 };
 
 /// Should be called to create batch payload object

lib/jsonrpc.js

@@ -45,7 +45,7 @@ var isValidResponse = function (response) {
         !response.error &&
         response.jsonrpc === '2.0' &&
         typeof response.id === 'number' &&
-        (!!response.result || typeof response.result === 'boolean');
+        response.result !== undefined; // only undefined is not valid json object
 };
 
 /// Should be called to create batch payload object

test/jsonrpc.isValidResponse.js

@@ -124,5 +124,20 @@ describe('jsonrpc', function () {
             assert.equal(valid, true);
         });
 
+        it('should validate jsonrpc response with result field === 0', function () {
+            
+            // given 
+            var response = {
+                jsonrpc: '2.0',
+                id: 1,
+                result: 0 
+            };
+
+            // when
+            var valid = jsonrpc.isValidResponse(response);
+
+            // then
+            assert.equal(valid, true);
+        });
     });
 });