Remove "stderr finished" garbage from end of request bodies

[josephmturner]

This issue pops up in older versions of Emacs.

The stderr process sentinel probably should be set to #'ignore.

[josephmturner]

Process plz-request-curl stderr<1> finished

appended to the end of each file

[alphapapa]

Being worked on in branch https://github.com/alphapapa/plz.el/tree/wip/no-mix-stderr. Solves the stated problem, but seems to introduce an intermittent error about "Selecting deleted buffer", and also sometimes prompts the user to delete a buffer that has a running process--and this happens despite safeguards in the code that should preclude such things. It seems that Emacs's process handling has race conditions, but I don't understand why they are being encountered in this library and not more widely among other Elisp libraries that use processes.

[alphapapa]

This should be solved in v0.7, and now fixed on master.