Update salesforce http header for Requests update

[dlstadther]

Description

With the release of Requests 2.11, headers were strictly required to contain string-only components. This requires the casting of Content-Length to the string equivalent of the length of the data.

Motivation and Context

Fix error caused by method check_header_validity in requests/utils.py (InvalidHeader: Header value 65 must be of type str or bytes, not <type 'int'>).

Requests Issue discussion

Have you tested this? If so, how?

Runs my jobs without error.

[mention-bot]

@dlstadther, thanks for your PR! By analyzing the history of the files in this pull request, we identified @dacort to be a potential reviewer.

[Tarrasch]

A test case would be good though.

[dlstadther]

I've finally gotten around to looking at testing this, but am unsure how to. The part that needs to be checked is that the header passed to requests is a dictionary of strings. Since the function does not simply return the response, but rather some portion of the parse response, I don't have access to that variable to check the header.

The other problem is fooling the function into thinking that I have a valid session and job_id.

Of course, I'll defer to you, @Tarrasch , if you really want me to move things around such that testing this is possible.

[Tarrasch]

No I'm fine with this. You can merge this I think. :)

[dlstadther]

Thanks for the approval @Tarrasch !