set blob as default XMLHttpRequest header response type if supported (facebook#22063)

Summary:
This is a problem that is discussed in issue facebook#21092

Related issues: facebook#21851 facebook#19717

Found the code to eventually fix this issue [here](https://github.com/github/fetch/blob/899b155746630c32d83ee29a38642da16b314ecb/fetch.js#L486)

- [x] Fetching an image locally and check if the blob is there, as well as its size > 0.

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[GENERAL] [ENHANCEMENT] [whatwg-fetch] - set blob as default XMLHttpRequest header response type if supported
Pull Request resolved: facebook#22063

Differential Revision: D13408797

Pulled By: cpojer

fbshipit-source-id: 9822d5a7e24bacd72838f3fc9a61b1a97b44484b

Author: reime005

Libraries/vendor/core/whatwg-fetch.js

@@ -516,6 +516,10 @@
         xhr.withCredentials = false;
       }
 
+      if ('responseType' in xhr && support.blob) {
+        xhr.responseType = 'blob'
+      }
+
       request.headers.forEach(function(value, name) {
         xhr.setRequestHeader(name, value);
       });