[4.0.0-alpha.0] fill doesn't work on MultiSelect, Suggest, Select

[danaugrs]

Package version(s): 4.0.0-alpha.0
This can be reproduced in the docs page: https://blueprintjs.com/docs/versions/4/#select/multi-select

It's possible that fill might also not work in Popover and InputGroup although I haven't tested (see #3636).

I upgraded to 4.0.0-alpha.0 in hopes that fill was implemented for Select but apparently it's not (see #2956).

PS: Blueprint is amazing. Thank you, to all the developers, maintainers and contributors!

[danaugrs]

Passing fill to both the Button inside the Select and to the Select's popoverProps seems to work for me without any CSS modifications, so it it's probably a simple fix.