Improvements to chunks coming from a stream/async reader (#348)

* fix the conditions triggering the warning about unexpected too large stream size
* Ability for chunks corresponding to stream, to be correctly retried if upload failed, #346

* tests:
- Improved the helpers related to file consistency and xhr final state
- Add tests for streamed-chunks retry, #346
- Jasmine does NOT support async + (done) tests (!). This is specifically forbidden in Jasmine 3.7. Workarounds must be used to use two distinct promises per tests (see jasmine/jasmine#1893)
- Handle case where an asyncReadFile function is used to process multiple Files
- Added a isReading() function + a (failing) function testing pause/resume() for streams
- Compute XHR upload checksum on successful XHR only.

Author: Raphaël Droz