Exposed filters values sometimes broken in the widget (displaying 1 instead of the actual value)

After the commit 2328c3c7 for issue #1261844: views_handler_filter_in_operator does not work with optgroups. (thanks a lot to git bisect !), exposed filters having some explicit value options and have the "Limit list to selected items" option checked generates broken options lists, where all the actual values are replaced by 1.

I'm attaching the export of a rather simple Views that demonstrate the problem.

The problem is the the form_options_flatten introduced to fix #1261844: views_handler_filter_in_operator does not work with optgroups. clobbers the actual values in $this->value_options, and replaces them by 1 whenever it's a simple key. As long as the values aren't used afterwards, that's ok-ish, but if the exposed filter is configured to Limit list to selected items, then those options are re-used for creating the actual widgets, leading to the broken display.

I'm working on a patch to fix that.