This forces _set_proxy_headers in urllib3.poolmanager.ProxyManager to
not send "Accept: */*" through the proxy. A proxy like mitmproxy will
pass along this "Accept" header to the destination, which is not always
the desired result.
Because of a bug in urllib3, requests==1.0.4 no longer works with
proxies. However, the adapter architecture was not around in 0.14.2, so
careful-requests has been refactored to work with either version.