Fix issue where download attribute is ignored.

See  https://code.google.com/p/chromium/issues/detail?id=375634 about the issue and the fix.

More about Blobs: https://developer.mozilla.org/en-US/docs/Web/API/Blob.
doing as advised.

advice is given here: https://developer.mozilla.org/en-US/docs/Web/API/URL.revokeObjectURL
Merge pull request #12 from NewCompte/fix-download-filename

Fix issue where filename is default.