Remove deprecated Buffer constructor usage

Fixes #1709

test/nodes/core/io/21-httprequest_spec.js

@@ -390,7 +390,7 @@ describe('HTTP Request Node', function() {
                         done(err);
                     }
                 });
-                n1.receive({payload:new Buffer('hello'), headers: { 'content-type': 'text/plain'}});
+                n1.receive({payload:Buffer.from('hello'), headers: { 'content-type': 'text/plain'}});
             });
         });
 

test/nodes/core/io/22-websocket_spec.js

@@ -454,7 +454,7 @@ describe('websocket Node', function() {
                 });
                 getSocket("n1").on("open", function() {
                     helper.getNode("n3").send({
-                        payload: new Buffer("hello")
+                        payload: Buffer.from("hello")
                     });
                 });
             });