Fixes#2192. Two problems here:
1. It's not documented but apparently we supported data frames for
choices in updateSelectInput/updateSelectizeInput (it doesn't
appear to work correctly for selectInput/selectizeInput though).
This was used in 023-optgroup-server as well as by the user who
reported #2172.
2. The example in 023-optgroup-server was also counting on the
default value of optgroupLabelField, which (starting post-Shiny
v1.1) was being set to a new default of "group". That now won't
happen unless optgroupField is also blank. I'm less confident
about the ramifications of this change. The selectize docs with
the relevant bits are here:
https://github.com/selectize/selectize.js/blob/master/docs/usage.md#data_searching