ox: Fix bug where properties read from setupfile overwrite previous properties
* lisp/ox.el (org-export--get-inbuffer-options): Remove an optional
argument. Rewrite function. Properties read from a setupfile do
not overwrite anymore previously computed properties.
(org-export-get-environment): Apply changes to previous function.
* lisp/org.el (org-create-formula--latex-header): Apply arity change
from `org-export--get-inbuffer-options'.
* testing/lisp/test-ox.el: Add test.
* testing/examples/setupfile.org: New file.