use Buffer global in http.js

Since the http adapter is never used in the browser it's safe to use
the Buffer global and its isBuffer() method directly.

lib/adapters/http.js

@@ -10,7 +10,6 @@ var httpsFollow = require('follow-redirects').https;
 var url = require('url');
 var zlib = require('zlib');
 var pkg = require('./../../package.json');
-var Buffer = require('buffer').Buffer;
 var createError = require('../core/createError');
 var enhanceError = require('../core/enhanceError');
 
@@ -30,7 +29,7 @@ module.exports = function httpAdapter(config) {
     }
 
     if (data && !utils.isStream(data)) {
-      if (utils.isBuffer(data)) {
+      if (Buffer.isBuffer(data)) {
         // Nothing to do...
       } else if (utils.isArrayBuffer(data)) {
         data = new Buffer(new Uint8Array(data));