fix buffer initialization in base64 encoding

If a number was passed to `encode()`, the buffer would be created with
uninitialised memory. This patch casts anything that's passed in to
a string first and then uses the safe `Buffer.from` API. `Buffer.from`
was added in Node v4 but the `buffer-from` module ponyfills it for older
Node versions.

See [nodejs/node#4660](nodejs/node#4660)

lib/node/base64.js

@@ -1,7 +1,7 @@
-/* global: Buffer */
+import bufferFrom from 'buffer-from';
 
 export function encode(data) {
-  return new Buffer(data).toString("base64");
+  return bufferFrom(String(data)).toString("base64");
 }
 
 export const isSupported = true;

package.json

@@ -54,6 +54,7 @@
     "watchify": "^3.5.0"
   },
   "dependencies": {
+    "buffer-from": "^0.1.1",
     "extend": "^3.0.0",
     "lodash.throttle": "^4.1.1",
     "resolve-url": "^0.2.1"