Use is-buffer instead of Buffer.isBuffer

The is-buffer module checks if an object is a Buffer without causing
Webpack or Browserify to include the whole Buffer module in the bundle.

package.json

@@ -72,6 +72,7 @@
   },
   "typings": "./index.d.ts",
   "dependencies": {
-    "follow-redirects": "^1.2.3"
+    "follow-redirects": "^1.2.3",
+    "is-buffer": "^1.1.5"
   }
 }