Retries during chunked transfers fail due to loss of chunked flag

[lmvlmv]

Possibly related to #1352

Using urllib via requests but this doesn't seem to be the cause. Recently, in requests, a fix was implemented to use the innate chunking capability in urllib.

If a custom Retry class is implemented, a chunked transfer is taking place and a retry-able condition occurs during the transfer the chunked flag is dropped when urlopen() is re-called during the retry.

In this case a generator was passed as the data argument and results in the non-chunking version of the code attempting to read it like raw data: TypeError: 'generator' object has no attribute 'len'

[lmvlmv]

Prototype fix here https://github.com/lmvlmv/urllib3/tree/chunkfix

Having trouble getting nox to pass tests in my environment before I submit as a PR...