Summary:After adding support for `XMLHttpRequest#response`, the `fetch` polyfill detects buffer support when debugging in Chrome and sets `responseType` to `'blob'`.
In that case, the response was always empty.
This change will construct a blob if `responseType` has been set to `'blob'` in order to avoid that problem
Reviewed By: steveluscher
Differential Revision: D3018884
fb-gh-sync-id: 4ade0413de67242c3565d95c2880d4a981ba2342
shipit-source-id: 4ade0413de67242c3565d95c2880d4a981ba2342