Fix sending requests with empty body such as DELETE in IE

If the body passed to `xhr.send()` is `undefined`, the text sent would
literally be "undefined" on IE.

fetch.js

@@ -202,6 +202,8 @@
       if (isObject(self.body)) {
         xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8')
         body = encode(self.body)
+      } else if (body === undefined) {
+        body = null
       }
       xhr.send(body)
     })