Filter terms based on "options_show_only_used_filtered" set not working as expected

#8 actually worked for me, even when the view is using ajax.

Hi guys, I finally found some free time and looked into the problem. I've reviewed this part of code and now looks like it solved and works. Only thing - it can't work with exposed filter in block with ajax, because view don't rebuild this block in this case. Thanks everybody for your comments and expiriencies, it was very useful.

Oh. This commit breaks working filters on my site

Same as #26. This commit breaks sites with only options_show_only_used set. I opened #3439538 to figure out a solution.