Return empty buffer upon empty response body and encoding is set to null

Instead of returning an empty string, an empty buffer will now be
returned when the encoding is set to null.

request.js

@@ -1321,7 +1321,7 @@ Request.prototype.onRequestResponse = function (response) {
         }
         debug('emitting complete', self.uri.href)
         if(typeof response.body === 'undefined' && !self._json) {
-          response.body = ''
+          response.body = self.encoding === null ? new Buffer(0) : ''
         }
         self.emit('complete', response, response.body)
       })

tests/test-emptyBody.js

@@ -15,7 +15,7 @@ tape('setup', function(t) {
   })
 })
 
-tape('empty body', function(t) {
+tape('empty body with encoding', function(t) {
   request('http://localhost:6767', function(err, res, body) {
     t.equal(err, null)
     t.equal(res.statusCode, 200)
@@ -24,6 +24,18 @@ tape('empty body', function(t) {
   })
 })
 
+tape('empty body without encoding', function(t) {
+  request({
+    url: 'http://localhost:6767',
+    encoding: null
+  }, function(err, res, body) {
+    t.equal(err, null)
+    t.equal(res.statusCode, 200)
+    t.same(body, new Buffer(0))
+    t.end()
+  })
+})
+
 tape('empty JSON body', function(t) {
   request({
     url: 'http://localhost:6767',