bug#23779: 25.0.95; consing "SHELLVAR" onto process-environment doesn't remove it from subprocess env

[Noam Postavsky <npostavs@users.sourceforge.net>]

Start emacs -Q, evaluate the following lisp code (I wrote the return
values after ;=>)

(defun check-env-var (var)
  (catch 'ret
    (dolist (var=val (process-lines "env"))
      (when (string-prefix-p var var=val)
        (throw 'ret var=val)))))

(check-env-var "SHELL");=>"SHELL=/bin/bash"
(let ((process-environment (copy-sequence process-environment)))
  (setenv "SHELL" nil)
  (check-env-var "SHELL"));=>nil
(let ((process-environment (cons "SHELL" process-environment)))
  (check-env-var "SHELL"));=>"SHELL=/bin/bash"
(let ((process-environment (cons "SHELL=" process-environment)))
  (check-env-var "SHELL"));=>"SHELL="

As you can see from the 3rd expression, contrary to its docstring,
consing the env variable "SHELL" onto process-environment has no
effect at all.

process-environment is a variable defined in ‘C source code’.
Its value is
[...]
Documentation:
List of overridden environment variables for subprocesses to inherit.
Each element should be a string of the form ENVVARNAME=VALUE.

Entries in this list take precedence to those in the frame-local
environments.  Therefore, let-binding ‘process-environment’ is an easy
way to temporarily change the value of an environment variable,
irrespective of where it comes from.  To use ‘process-environment’ to
remove an environment variable, include only its name in the list,
without "=VALUE".