Declare deliberate "async=true" on XMLHttpRequest open

On some installs, both Firefox & Chrome throws:
"Fetch failed: " DOMException [InvalidAccessError]: "A parameter or an operation is not supported by the underlying object"
Due to pre-set async=false (overridden prototype, maybe?)
This fixes the above issue.

fetch.js

@@ -274,7 +274,7 @@
         reject(new TypeError('Network request failed'))
       }
 
-      xhr.open(self.method, self.url)
+      xhr.open(self.method, self.url, true)
       if ('responseType' in xhr && blobSupport) {
         xhr.responseType = 'blob'
       }