…e out-of-the-box

…total amount of bytes to upload.

The number of chunks may also be off. Let fail safely:
- if read using an async function
- and the last chunk read less bytes than usual (including 0)
- and the current chunk read no byte
- assume that this chunk is in fact superfluous and simulate its completion.

Previous discussion on:
flowjs#42
flowjs#318

In the long-term Flow.js may have to support the concept of "approximate chunk number".