remove _send_request_safe_mode

[kevinburke]

From the code:

    This is equivalent of python-requests' safe_mode, which due to a bug, does currently *not*
    work together with Sessions. Once the issue is fixed in python-requests, this method should
    be removed. See: https://github.com/kennethreitz/requests/issues/888

It looks like https://github.com/kennethreitz/requests/pull/953 fixed the issue, so this method is no longer necessary.

[heyman]

It's fixed, but unfortunately I don't think the bugfix was ever released in a 0.14.x bugfix release, which is the version that Locust is currently depending on. Once we've fixed support for the newest version of requests (hopefully soon) we will be able to remove it :).