2d1a605 - Backport HTTPS proxy fix really needed

[Rostislav Svoboda]

The '2d1a605 - Backport HTTPS proxy fix' really fixes that https proxy bug.
I just tested that. Unfortunatelly Lars reverted it in 0d7aad1 stating:
    It's too late in the Emacs 25 release cycle to add things like this to
    Emacs 25.1.  It's border line new feature.

I object. It's not a new-feature-coming-too-late it's a commit fixing
real problem!
Without it no package upgrade via https works for me. Only via http.
Please DO include the 2d1a605 in the emacs-25

thx Bost